wip

.env

@@ -1,2 +1,15 @@
 # This file is used by dbmate, and by the sqlx macros at compile time.
 DATABASE_URL=postgres://postgres:postgres@localhost:5432/cargagep?sslmode=disable
+
+# Read by the backend too: anything named APP__* here reaches the configuration
+# exactly like an exported variable would, and a real environment variable still
+# wins over this file. This is where the secrets live in development.
+#
+# The bot that posts to the cargobikes group. Leave both empty and nothing is
+# sent — which is what a machine without the bot wants.
+#   * BOT_TOKEN comes from @BotFather
+#   * CHAT_ID is the group's id: add the bot to the group, post a message, then
+#     read it from https://api.telegram.org/bot<token>/getUpdates (a group id is
+#     negative, e.g. -1001234567890)
+APP__TELEGRAM__BOT_TOKEN=
+APP__TELEGRAM__CHAT_ID=

db/migrations/20260824180000_reservation_bike_periods.sql

@@ -0,0 +1,25 @@
+-- migrate:up
+
+-- A bike is normally held for its reservation's own period. These two columns
+-- override it for one bike, which is how a conflict is resolved without moving
+-- the whole booking: the reservation keeps its hours, and only the bike both
+-- bookings want changes hands earlier.
+--
+-- NULL means "the reservation's period", so every existing row keeps behaving
+-- exactly as before.
+ALTER TABLE reservations_bikes
+    ADD COLUMN start_time timestamptz,
+    ADD COLUMN end_time timestamptz;
+
+-- Both or neither, and the right way round
+ALTER TABLE reservations_bikes ADD CONSTRAINT reservations_bikes_period CHECK (
+    (start_time IS NULL AND end_time IS NULL)
+    OR (start_time IS NOT NULL AND end_time IS NOT NULL AND start_time < end_time));
+
+-- Conflicts are looked up by bike — "who else holds this one, and when" — which
+-- `reservations_bikes_bike_id_idx` already serves: it was created with the
+-- table, and the primary key, starting with reservation_id, could not.
+
+-- migrate:down
+ALTER TABLE reservations_bikes DROP CONSTRAINT reservations_bikes_period;
+ALTER TABLE reservations_bikes DROP COLUMN start_time, DROP COLUMN end_time;

db/schema.sql

@@ -130,7 +130,10 @@ CREATE TABLE public.reservations (
 
 CREATE TABLE public.reservations_bikes (
     reservation_id integer NOT NULL,
-    bike_id integer NOT NULL
+    bike_id integer NOT NULL,
+    start_time timestamp with time zone,
+    end_time timestamp with time zone,
+    CONSTRAINT reservations_bikes_period CHECK ((((start_time IS NULL) AND (end_time IS NULL)) OR ((start_time IS NOT NULL) AND (end_time IS NOT NULL) AND (start_time < end_time))))
 );
 
 
@@ -505,4 +508,5 @@ INSERT INTO public.schema_migrations (version) VALUES
     ('20260823210000'),
     ('20260823230000'),
     ('20260824120000'),
-    ('20260824140000');
+    ('20260824140000'),
+    ('20260824180000');

db/seed.sql

@@ -81,18 +81,41 @@ FROM (VALUES
      date_trunc('day', now()) + interval '6 days' + interval '18 hours',
      2, '@bob_dupont', 'Annulée faute de conducteur',
      ARRAY['bob.dupont@epfl.ch'],
-     'cancelled'::reservation_status)
+     'cancelled'::reservation_status),
+    -- Wants the 5000 while reservation 2 still holds it, so the admin page has
+    -- a conflict to warn about without anybody having to build one by hand
+    (7, 'S4S',
+     date_trunc('day', now()) + interval '1 day' + interval '14 hours',
+     date_trunc('day', now()) + interval '2 days' + interval '10 hours',
+     1, '@alice_martin', 'Stand de la journée portes ouvertes',
+     ARRAY['alice.martin@epfl.ch'],
+     'requested'::reservation_status)
 ) AS r (id, unit_name, start_time, end_time, requester_id, telegram, "description",
           linka_emails, status)
 JOIN public.units u ON u."name" = r.unit_name
 ON CONFLICT DO NOTHING;
 
 INSERT INTO public.reservations_users (reservation_id, user_id) VALUES
-    (1, 1), (1, 2), (2, 3), (3, 2), (3, 1), (4, 3), (5, 1), (6, 2)
+    (1, 1), (1, 2), (2, 3), (3, 2), (3, 1), (4, 3), (5, 1), (6, 2), (7, 1)
 ON CONFLICT DO NOTHING;
 
-INSERT INTO public.reservations_bikes (reservation_id, bike_id) VALUES
-    (1, 1), (1, 2), (2, 3), (3, 1), (3, 4), (4, 2), (5, 5), (6, 4)
+-- The bikes each reservation holds. The two period columns are normally NULL,
+-- which means "the reservation's own period"; they are only filled when a bike
+-- is handed over early to settle a conflict, as reservation 1 does with the
+-- 2000 — that one is given back at noon today rather than tomorrow evening.
+INSERT INTO public.reservations_bikes (reservation_id, bike_id, start_time, end_time) VALUES
+    (1, 1, NULL::timestamptz, NULL::timestamptz),
+    (1, 2, date_trunc('day', now()) - interval '1 day' + interval '8 hours',
+           date_trunc('day', now()) + interval '12 hours'),
+    (2, 3, NULL, NULL),
+    -- Held by reservation 2, and wanted by reservation 7 at the same time
+    (2, 5, NULL, NULL),
+    (3, 1, NULL, NULL),
+    (3, 4, NULL, NULL),
+    (4, 2, NULL, NULL),
+    (5, 5, NULL, NULL),
+    (6, 4, NULL, NULL),
+    (7, 5, NULL, NULL)
 ON CONFLICT DO NOTHING;
 
 -- Keep the sequences in sync with the explicit ids inserted above

frontend/src/components/admin/BikeFleet.vue

@@ -26,8 +26,14 @@ const setStatus = useSetBikeStatus()
 /** Bikes held by a reservation that is under way right now */
 const inUse = computed(() => {
   const ids = new Set<number>()
+  const now = Date.now()
   for (const reservation of props.reservations) {
-    if (reservation.status === 'ongoing') reservation.bikes.forEach((id) => ids.add(id))
+    if (reservation.status !== 'ongoing') continue
+    // A bike handed over early is no longer in use, even though the
+    // reservation it belonged to is still running
+    for (const bike of reservation.bikes) {
+      if (new Date(bike.end_time).getTime() > now) ids.add(bike.id)
+    }
   }
   return ids
 })

frontend/src/components/admin/ReservationCard.vue

@@ -6,15 +6,23 @@
  */
 import { computed, ref } from 'vue'
 import { useI18n } from 'vue-i18n'
-import { Pencil } from '@lucide/vue'
+import { Pencil, TriangleAlert } from '@lucide/vue'
 import { toast } from 'vue-sonner'
 
 import ReservationDetails from '@/components/reservation/ReservationDetails.vue'
 import ReservationEditDialog from '@/components/reservation/ReservationEditDialog.vue'
 import { Badge } from '@/components/ui/badge'
 import { Button } from '@/components/ui/button'
-import { useSetReservationStatus } from '@/services/api/reservations'
-import { unitLabel, type Bike, type Reservation, type ReservationStatus } from '@/utils/types'
+import { useConflicts, useSetReservationStatus } from '@/services/api/reservations'
+import { HttpStatus } from 'http-status-ts'
+
+import {
+  ApiError,
+  unitLabel,
+  type Bike,
+  type Reservation,
+  type ReservationStatus,
+} from '@/utils/types'
 
 const props = defineProps<{ reservation: Reservation; bikes: Bike[] }>()
 
@@ -44,6 +52,47 @@ const BADGE_CLASS: Record<ReservationStatus, string> = {
 
 const transitions = computed(() => TRANSITIONS[props.reservation.status])
 
+/**
+ * What approving this reservation would clash with.
+ *
+ * Only asked while approving is on the table: an already approved one has
+ * taken its bikes, and a final one has given them back. The backend refuses
+ * the transition too — this is what says why, before the click.
+ */
+const canApprove = computed(() => transitions.value.includes('approved'))
+const probe = computed(() =>
+  canApprove.value
+    ? {
+        reservation: props.reservation.id,
+        start_time: props.reservation.start_time,
+        end_time: props.reservation.end_time,
+        bikes: props.reservation.bikes.map((held) => ({
+          id: held.id,
+          start_time: held.start_time,
+          end_time: held.end_time,
+        })),
+      }
+    : null,
+)
+const { data: conflicts } = useConflicts(probe, { enabled: canApprove })
+
+const blocking = computed(() =>
+  (conflicts.value ?? []).map((conflict) => ({
+    ...conflict,
+    name: props.bikes.find((bike) => bike.id === conflict.bike)?.name ?? `#${conflict.bike}`,
+  })),
+)
+
+const { locale } = useI18n()
+const formatter = computed(
+  () =>
+    new Intl.DateTimeFormat(locale.value === 'fr' ? 'fr-CH' : 'en-GB', {
+      dateStyle: 'medium',
+      timeStyle: 'short',
+    }),
+)
+const format = (iso: string) => formatter.value.format(new Date(iso))
+
 /** A reservation nobody can act on any more is not worth an edit button */
 const editable = computed(
   () => !['refused', 'cancelled', 'archived'].includes(props.reservation.status),
@@ -51,9 +100,18 @@ const editable = computed(
 const editing = ref(false)
 
 function move(status: ReservationStatus) {
+  // The button is disabled while a conflict stands, but the list it was drawn
+  // from may be a few seconds old: the backend has the last word.
   setStatus.mutate(
     { id: props.reservation.id, status },
-    { onError: () => toast.error(t('admin.reservations.error')) },
+    {
+      onError: (error) =>
+        toast.error(
+          error instanceof ApiError && error.status === HttpStatus.CONFLICT
+            ? t('admin.reservations.conflict.error')
+            : t('admin.reservations.error'),
+        ),
+    },
   )
 }
 </script>
@@ -79,7 +137,12 @@ function move(status: ReservationStatus) {
           :key="status"
           size="sm"
           :variant="status === 'approved' ? 'default' : 'outline'"
-          :disabled="setStatus.isPending.value"
+          :disabled="setStatus.isPending.value || (status === 'approved' && blocking.length > 0)"
+          :title="
+            status === 'approved' && blocking.length
+              ? $t('admin.reservations.conflict.title')
+              : undefined
+          "
           @click="move(status)"
         >
           {{ $t(`admin.reservations.action.${status}`) }}
@@ -87,6 +150,33 @@ function move(status: ReservationStatus) {
       </div>
     </div>
 
+    <!-- Why the reservation cannot be approved, named rather than merely refused -->
+    <div
+      v-if="blocking.length"
+      class="border-destructive/40 bg-destructive/5 mt-3 grid gap-1 rounded-md border p-3"
+    >
+      <p class="text-destructive flex items-center gap-2 text-sm font-medium">
+        <TriangleAlert class="size-4 shrink-0" />
+        {{ $t('admin.reservations.conflict.title') }}
+      </p>
+      <p
+        v-for="conflict in blocking"
+        :key="`${conflict.bike}-${conflict.reservation}`"
+        class="text-sm"
+      >
+        {{
+          $t('admin.reservations.conflict.line', {
+            bike: conflict.name,
+            id: conflict.reservation,
+            unit: conflict.unit,
+            from: format(conflict.start_time),
+            to: format(conflict.end_time),
+          })
+        }}
+      </p>
+      <p class="text-muted-foreground text-xs">{{ $t('admin.reservations.conflict.hint') }}</p>
+    </div>
+
     <ReservationDetails class="mt-3 border-t pt-3" :reservation="reservation" :bikes="bikes" />
 
     <ReservationEditDialog

frontend/src/components/reservation/BikeCalendar.vue

@@ -114,11 +114,13 @@ function blocksFor(day: Date, bike: Bike): Block[] {
   const dayEnd = new Date(day)
   dayEnd.setHours(LAST_HOUR, 0, 0, 0)
 
-  return booked.value
-    .filter((reservation) => reservation.bikes.includes(bike.id))
-    .flatMap((reservation) => {
-      const start = new Date(reservation.start_time)
-      const end = new Date(reservation.end_time)
+  return booked.value.flatMap((reservation) => {
+    // The bike's own period, which is not always the reservation's: a
+    // conflict may have been settled by handing this one over early
+    const held = reservation.bikes.find((held) => held.id === bike.id)
+    if (!held) return []
+    const start = new Date(held.start_time)
+    const end = new Date(held.end_time)
     const from = start > dayStart ? start : dayStart
     const to = end < dayEnd ? end : dayEnd
     if (to <= from) return []

frontend/src/components/reservation/ReservationDetails.vue

@@ -34,10 +34,19 @@ function shows(field: Field) {
 const { locale } = useI18n()
 
 /** Names when we know the bike, `#id` when the list has not loaded yet */
-const bikeNames = computed(() =>
-  props.reservation.bikes.map((id) => props.bikes.find((bike) => bike.id === id)?.name ?? `#${id}`),
+function bikeName(id: number) {
+  return props.bikes.find((bike) => bike.id === id)?.name ?? `#${id}`
+}
+
+const heldBikes = computed(() =>
+  props.reservation.bikes.map((held) => ({ ...held, name: bikeName(held.id) })),
 )
 
+// A bike whose period is not the reservation's gets a line of its own: it is
+// the whole point of the override, and hiding it would make the reservation
+// look like it still holds the bike to the end.
+const rescheduled = computed(() => heldBikes.value.filter((held) => held.custom))
+
 // The e-mail address is already on the reservation as a Linka Go account or in
 // the admin's own directory: the name is what tells the people apart here.
 const peopleNames = computed(() =>
@@ -77,17 +86,28 @@ function format(iso: string) {
       </dd>
     </div>
 
-    <div v-if="shows('bikes') && bikeNames.length" class="min-w-0">
+    <div v-if="shows('bikes') && heldBikes.length" class="min-w-0">
       <dt class="text-muted-foreground text-xs font-medium tracking-wide uppercase">
         {{ $t('reservation.details.bikes') }}
       </dt>
       <dd class="mt-0.5 flex flex-wrap gap-1">
         <span
-          v-for="name in bikeNames"
-          :key="name"
-          class="bg-muted rounded-md px-2 py-0.5 font-medium tabular-nums"
+          v-for="held in heldBikes"
+          :key="held.id"
+          class="rounded-md px-2 py-0.5 font-medium tabular-nums"
+          :class="held.custom ? 'bg-amber-500/15 text-amber-700 dark:text-amber-400' : 'bg-muted'"
+          :title="held.custom ? $t('reservation.details.own-period') : undefined"
         >
-          {{ name }}
+          {{ held.name }}
+        </span>
+      </dd>
+      <dd v-if="rescheduled.length" class="mt-1 grid gap-0.5">
+        <span
+          v-for="held in rescheduled"
+          :key="held.id"
+          class="text-muted-foreground text-xs tabular-nums"
+        >
+          {{ held.name }} · {{ format(held.start_time) }} → {{ format(held.end_time) }}
         </span>
       </dd>
     </div>

frontend/src/components/reservation/ReservationEditDialog.vue

@@ -11,9 +11,11 @@
  */
 import { computed, reactive, ref, shallowRef, watch } from 'vue'
 import { useI18n } from 'vue-i18n'
-import { Plus, X } from '@lucide/vue'
+import { refDebounced } from '@vueuse/core'
+import { CalendarClock, Plus, TriangleAlert, Undo2, X } from '@lucide/vue'
 import { CalendarDate, getLocalTimeZone, type DateValue } from '@internationalized/date'
 import { toast } from 'vue-sonner'
+import { HttpStatus } from 'http-status-ts'
 
 import DatePicker from '@/components/DatePicker.vue'
 import TimePicker from '@/components/TimePicker.vue'
@@ -30,8 +32,15 @@ import {
 } from '@/components/ui/dialog'
 import { Input } from '@/components/ui/input'
 import { Label } from '@/components/ui/label'
-import { useUpdateReservation } from '@/services/api/reservations'
-import { unitLabel, type Bike, type Reservation } from '@/utils/types'
+import { useConflicts, useUpdateReservation } from '@/services/api/reservations'
+import {
+  ApiError,
+  unitLabel,
+  type Bike,
+  type Conflict,
+  type NewReservationBike,
+  type Reservation,
+} from '@/utils/types'
 
 const props = withDefaults(
   defineProps<{ reservation: Reservation; bikes: Bike[]; emailsOnly?: boolean }>(),
@@ -39,7 +48,7 @@ const props = withDefaults(
 )
 const open = defineModel<boolean>('open', { default: false })
 
-const { t } = useI18n()
+const { t, locale } = useI18n()
 const update = useUpdateReservation()
 
 const EMAIL_RE = /^[^\s@]+@[^\s@]+\.[^\s@]+$/
@@ -56,6 +65,15 @@ const form = reactive({
 const errors = reactive<Record<string, string>>({})
 const submitted = ref(false)
 
+/**
+ * A period one bike is held for, when it is not the reservation's own. This is
+ * how a conflict is settled: the booking keeps its hours and the disputed bike
+ * changes hands earlier. A bike absent from here simply follows the
+ * reservation.
+ */
+type Period = { startDate: DateValue; startTime: string; endDate: DateValue; endTime: string }
+const overrides = reactive<Record<number, Period>>({})
+
 function toCalendarDate(date: Date): DateValue {
   return new CalendarDate(date.getFullYear(), date.getMonth() + 1, date.getDate())
 }
@@ -67,6 +85,17 @@ function toSlot(date: Date): string {
   return `${String(Math.floor(total / 60)).padStart(2, '0')}:${String(total % 60).padStart(2, '0')}`
 }
 
+function toPeriod(from: string, to: string): Period {
+  const start = new Date(from)
+  const end = new Date(to)
+  return {
+    startDate: toCalendarDate(start),
+    startTime: toSlot(start),
+    endDate: toCalendarDate(end),
+    endTime: toSlot(end),
+  }
+}
+
 /** Reloads the fields from the reservation, so cancelling really cancels */
 function load() {
   const start = new Date(props.reservation.start_time)
@@ -75,7 +104,11 @@ function load() {
   endDate.value = toCalendarDate(end)
   form.startTime = toSlot(start)
   form.endTime = toSlot(end)
-  form.bikes = [...props.reservation.bikes]
+  form.bikes = props.reservation.bikes.map((held) => held.id)
+  Object.keys(overrides).forEach((key) => delete overrides[Number(key)])
+  for (const held of props.reservation.bikes) {
+    if (held.custom) overrides[held.id] = toPeriod(held.start_time, held.end_time)
+  }
   form.emails = props.reservation.linka_emails.length ? [...props.reservation.linka_emails] : ['']
   Object.keys(errors).forEach((key) => delete errors[key])
   submitted.value = false
@@ -104,16 +137,118 @@ const end = computed(() => toDate(endDate.value, form.endTime))
  * of service — the same rule the backend applies.
  */
 function isBlocked(bike: Bike) {
-  return bike.status === 'out_of_service' && !props.reservation.bikes.includes(bike.id)
+  return (
+    bike.status === 'out_of_service' && !props.reservation.bikes.some((held) => held.id === bike.id)
+  )
 }
 
 function toggleBike(bike: Bike) {
   if (isBlocked(bike)) return
   const index = form.bikes.indexOf(bike.id)
-  if (index >= 0) form.bikes.splice(index, 1)
-  else form.bikes.push(bike.id)
+  if (index >= 0) {
+    form.bikes.splice(index, 1)
+    delete overrides[bike.id]
+  } else form.bikes.push(bike.id)
 }
 
+function bikeName(id: number) {
+  return props.bikes.find((bike) => bike.id === id)?.name ?? `#${id}`
+}
+
+/** The period a bike would be held for once saved, reservation's or its own */
+function heldPeriod(id: number): [Date, Date] | null {
+  const own = overrides[id]
+  const from = own ? toDate(own.startDate, own.startTime) : start.value
+  const to = own ? toDate(own.endDate, own.endTime) : end.value
+  return from && to ? [from, to] : null
+}
+
+function useOwnPeriod(id: number) {
+  if (!start.value || !end.value) return
+  overrides[id] = toPeriod(start.value.toISOString(), end.value.toISOString())
+}
+
+function followReservation(id: number) {
+  delete overrides[id]
+}
+
+/** The fleet in the shape the backend takes: no period means "the booking's" */
+const editedBikes = computed<NewReservationBike[]>(() =>
+  form.bikes.map((id) => {
+    const period = heldPeriod(id)
+    if (!overrides[id] || !period) return { id }
+    return { id, start_time: period[0].toISOString(), end_time: period[1].toISOString() }
+  }),
+)
+
+/**
+ * What the reservation would clash with once saved. Answered by the backend
+ * against the whole table, and only a warning: whoever manages the reservation
+ * is allowed to double-book knowingly, and is the only one who can sort it out.
+ */
+const probe = computed(() => {
+  // Emails-only mode changes neither the period nor the fleet: nothing to check
+  if (props.emailsOnly || !start.value || !end.value || !form.bikes.length) return null
+  if (end.value <= start.value) return null
+  return {
+    reservation: props.reservation.id,
+    start_time: start.value.toISOString(),
+    end_time: end.value.toISOString(),
+    bikes: editedBikes.value,
+  }
+})
+// Every picker change would otherwise be its own request
+const debouncedProbe = refDebounced(probe, 300)
+const { data: conflicts } = useConflicts(debouncedProbe, { enabled: open })
+
+/** One block per bike, since the same one can be held by two reservations */
+const conflictsByBike = computed(() => {
+  const grouped = new Map<number, Conflict[]>()
+  for (const conflict of conflicts.value ?? []) {
+    const list = grouped.get(conflict.bike)
+    if (list) list.push(conflict)
+    else grouped.set(conflict.bike, [conflict])
+  }
+  return [...grouped].map(([id, list]) => ({ id, name: bikeName(id), conflicts: list }))
+})
+
+/**
+ * The one-click way out, when there is one: hand the bike back before the other
+ * booking takes it, or take it once that one is done. It only ever shortens our
+ * own hold, so it is always safe — and it is exactly what settles the case
+ * where two bookings want the same bike on either side of an hour.
+ */
+function resolution(id: number, conflict: Conflict): { at: Date; kind: 'end' | 'start' } | null {
+  const period = heldPeriod(id)
+  if (!period) return null
+  const [from, to] = period
+  const otherFrom = new Date(conflict.start_time)
+  const otherTo = new Date(conflict.end_time)
+  if (otherFrom > from) return { at: otherFrom, kind: 'end' }
+  if (otherTo < to) return { at: otherTo, kind: 'start' }
+  // The other booking covers ours whole: only moving the reservation, or the
+  // other one, can settle this
+  return null
+}
+
+function applyResolution(id: number, conflict: Conflict) {
+  const fix = resolution(id, conflict)
+  const period = heldPeriod(id)
+  if (!fix || !period) return
+  const [from, to] = period
+  const next = fix.kind === 'end' ? [from, fix.at] : [fix.at, to]
+  overrides[id] = toPeriod(next[0].toISOString(), next[1].toISOString())
+}
+
+const timeFormatter = computed(
+  () =>
+    new Intl.DateTimeFormat(locale.value === 'fr' ? 'fr-CH' : 'en-GB', {
+      dateStyle: 'medium',
+      timeStyle: 'short',
+    }),
+)
+const formatMoment = (value: Date | string) => timeFormatter.value.format(new Date(value))
+
 function addEmail() {
   form.emails.push('')
 }
@@ -133,6 +268,20 @@ function validate(): boolean {
       errors.end = t('reservation.error-end-before-start')
     }
     if (form.bikes.length === 0) errors.bikes = t('reservation.error-no-bike')
+
+    // The backend refuses these too; saying so here saves a round trip and
+    // names the bike rather than the whole reservation
+    for (const id of form.bikes) {
+      const own = overrides[id]
+      if (!own) continue
+      const from = toDate(own.startDate, own.startTime)
+      const to = toDate(own.endDate, own.endTime)
+      if (!from || !to || to <= from) {
+        errors.periods = t('reservation.edit.error-period', { bike: bikeName(id) })
+      } else if (start.value && end.value && (from < start.value || to > end.value)) {
+        errors.periods = t('reservation.edit.error-period-outside', { bike: bikeName(id) })
+      }
+    }
   }
 
   const emails = form.emails.map((email) => email.trim()).filter(Boolean)
@@ -158,7 +307,12 @@ function save() {
       // the backend checks before accepting the change
       start_time: props.emailsOnly ? props.reservation.start_time : start.value!.toISOString(),
       end_time: props.emailsOnly ? props.reservation.end_time : end.value!.toISOString(),
-      bikes: props.emailsOnly ? [...props.reservation.bikes] : [...form.bikes],
+      bikes: props.emailsOnly
+        ? props.reservation.bikes.map((held) => ({
+            id: held.id,
+            ...(held.custom ? { start_time: held.start_time, end_time: held.end_time } : {}),
+          }))
+        : editedBikes.value,
       linka_emails: form.emails.map((email) => email.trim()).filter(Boolean),
     },
     {
@@ -166,7 +320,12 @@ function save() {
         toast.success(t('reservation.edit.saved'))
         open.value = false
       },
-      onError: (error) => toast.error(error.message || t('reservation.edit.error')),
+      onError: (error) =>
+        toast.error(
+          error instanceof ApiError && error.status === HttpStatus.CONFLICT
+            ? t('reservation.conflicts.refused')
+            : error.message || t('reservation.edit.error'),
+        ),
     },
   )
 }
@@ -254,6 +413,111 @@ function save() {
           <p v-if="errors.bikes" class="text-destructive text-xs">{{ errors.bikes }}</p>
         </div>
 
+        <!-- One period per bike, which is how a conflict gets settled without
+             moving the whole booking -->
+        <div v-if="!emailsOnly && form.bikes.length" class="grid gap-2">
+          <span class="text-sm font-medium">{{ $t('reservation.edit.periods') }}</span>
+          <div v-for="id in form.bikes" :key="id" class="grid gap-2 rounded-md border p-3">
+            <div class="flex flex-wrap items-center justify-between gap-2">
+              <span class="flex items-center gap-2 text-sm font-medium tabular-nums">
+                {{ bikeName(id) }}
+                <span v-if="!overrides[id]" class="text-muted-foreground font-normal">
+                  {{ $t('reservation.edit.follows') }}
+                </span>
+              </span>
+              <Button
+                v-if="!overrides[id]"
+                type="button"
+                variant="ghost"
+                size="sm"
+                @click="useOwnPeriod(id)"
+              >
+                <CalendarClock class="size-4" />
+                {{ $t('reservation.edit.own-period') }}
+              </Button>
+              <Button v-else type="button" variant="ghost" size="sm" @click="followReservation(id)">
+                <Undo2 class="size-4" />
+                {{ $t('reservation.edit.follow-again') }}
+              </Button>
+            </div>
+
+            <div v-if="overrides[id]" class="grid gap-2">
+              <span class="text-muted-foreground text-xs">
+                {{ $t('reservation.edit.period-from') }}
+              </span>
+              <div class="grid gap-2 sm:grid-cols-2">
+                <DatePicker
+                  v-model="overrides[id].startDate"
+                  :min-value="startDate"
+                  :max-value="endDate"
+                />
+                <TimePicker v-model="overrides[id].startTime" />
+              </div>
+              <span class="text-muted-foreground text-xs">
+                {{ $t('reservation.edit.period-to') }}
+              </span>
+              <div class="grid gap-2 sm:grid-cols-2">
+                <DatePicker
+                  v-model="overrides[id].endDate"
+                  :min-value="overrides[id].startDate"
+                  :max-value="endDate"
+                />
+                <TimePicker v-model="overrides[id].endTime" />
+              </div>
+            </div>
+          </div>
+          <p v-if="errors.periods" class="text-destructive text-xs">{{ errors.periods }}</p>
+        </div>
+
+        <!-- A warning, not a wall: whoever manages the reservation is allowed to
+             double-book on purpose, and is the only one who can settle it -->
+        <div
+          v-if="conflictsByBike.length"
+          class="grid gap-2 rounded-md border border-amber-500/40 bg-amber-500/10 p-3"
+        >
+          <p class="flex items-center gap-2 text-sm font-medium text-amber-700 dark:text-amber-400">
+            <TriangleAlert class="size-4 shrink-0" />
+            {{ $t('reservation.conflicts.title') }}
+          </p>
+          <p class="text-muted-foreground text-xs">{{ $t('reservation.conflicts.intro') }}</p>
+          <div v-for="group in conflictsByBike" :key="group.id" class="grid gap-1">
+            <div v-for="conflict in group.conflicts" :key="conflict.reservation" class="text-sm">
+              <p>
+                <span class="font-medium tabular-nums">{{ group.name }}</span>
+                —
+                {{
+                  $t('reservation.conflicts.held', {
+                    unit: conflict.unit,
+                    id: conflict.reservation,
+                    from: formatMoment(conflict.start_time),
+                    to: formatMoment(conflict.end_time),
+                  })
+                }}
+              </p>
+              <Button
+                v-if="resolution(group.id, conflict)"
+                type="button"
+                variant="outline"
+                size="sm"
+                class="mt-1"
+                @click="applyResolution(group.id, conflict)"
+              >
+                {{
+                  $t(
+                    resolution(group.id, conflict)!.kind === 'end'
+                      ? 'reservation.conflicts.give-back'
+                      : 'reservation.conflicts.take-later',
+                    {
+                      bike: group.name,
+                      at: formatMoment(resolution(group.id, conflict)!.at),
+                    },
+                  )
+                }}
+              </Button>
+            </div>
+          </div>
+        </div>
+
         <!-- Linka Go accounts -->
         <div class="grid gap-2">
           <Label for="edit-email-0">{{ $t('reservation.emails') }}</Label>

frontend/src/lib/api.d.ts

@@ -512,7 +512,7 @@ export interface paths {
           }
           content?: never
         }
-        /** @description One of the bikes is out of service */
+        /** @description One of the bikes is out of service, or is already held over that period */
         409: {
           headers: {
             [name: string]: unknown
@@ -644,6 +644,66 @@ export interface paths {
     patch?: never
     trace?: never
   }
+  '/api/reservations/conflicts': {
+    parameters: {
+      query?: never
+      header?: never
+      path?: never
+      cookie?: never
+    }
+    get?: never
+    put?: never
+    /**
+     * Which of the bikes wanted are already held over the same period
+     * @description Only approved and ongoing reservations hold a bike, so only they appear here. The booking form uses it to grey out what is taken, and the admin page to warn before double-booking on purpose.
+     */
+    post: {
+      parameters: {
+        query?: never
+        header?: never
+        path?: never
+        cookie?: never
+      }
+      /**
+       * @description What a would-be booking asks about: a period, the bikes wanted, and the
+       *      reservation to leave out of the answer when one is being edited.
+       */
+      requestBody: {
+        content: {
+          'application/json': components['schemas']['ConflictProbeForm']
+        }
+      }
+      responses: {
+        200: {
+          headers: {
+            [name: string]: unknown
+          }
+          content: {
+            'application/json': components['schemas']['Conflict'][]
+          }
+        }
+        /** @description The period is empty or the wrong way round */
+        400: {
+          headers: {
+            [name: string]: unknown
+          }
+          content?: never
+        }
+        /** @description Unauthenticated - a session is required */
+        401: {
+          headers: {
+            [name: string]: unknown
+          }
+          content?: never
+        }
+      }
+    }
+    delete?: never
+    options?: never
+    head?: never
+    patch?: never
+    trace?: never
+  }
   '/api/reservations/{id}': {
     parameters: {
       query?: never
@@ -712,7 +772,7 @@ export interface paths {
           }
           content?: never
         }
-        /** @description A newly added bike is out of service, or the reservation is final */
+        /** @description A newly added bike is out of service or already held over that period (managers may double-book on purpose, so this only reaches anybody else), or the reservation is final */
         409: {
           headers: {
             [name: string]: unknown
@@ -837,7 +897,8 @@ export interface components {
      *      travels to anybody, signed in or not.
      */
     CalendarReservation: {
-      bikes: number[]
+      /** @description With their own periods: a bike handed over early is drawn as such */
+      bikes: components['schemas']['ReservationBike'][]
       /** Format: date-time */
       end_time: string
       /** Format: int32 */
@@ -858,6 +919,43 @@ export interface components {
       /** Format: date-time */
       to?: string | null
     }
+    /**
+     * @description A bike the probe wants that somebody else already holds over part of the
+     *      same period. Only the reservations that actually hold a bike — approved and
+     *      ongoing — can produce one.
+     */
+    Conflict: {
+      /** Format: int32 */
+      bike: number
+      /** Format: date-time */
+      end_time: string
+      /** Format: int32 */
+      reservation: number
+      /**
+       * Format: date-time
+       * @description When the bike is held, which is what a resolution has to step around
+       */
+      start_time: string
+      status: components['schemas']['ReservationStatus']
+      /** @description Who holds it, to be named in the warning */
+      unit: string
+    }
+    /**
+     * @description What a would-be booking asks about: a period, the bikes wanted, and the
+     *      reservation to leave out of the answer when one is being edited.
+     */
+    ConflictProbeForm: {
+      bikes: components['schemas']['NewReservationBike'][]
+      /** Format: date-time */
+      end_time: string
+      /**
+       * Format: int32
+       * @description The reservation being edited, so it does not conflict with itself
+       */
+      reservation?: number | null
+      /** Format: date-time */
+      start_time: string
+    }
     DevUser: {
       admin: boolean
       email: string
@@ -887,6 +985,18 @@ export interface components {
       unit: components['schemas']['NewReservationUnit']
       users: number[]
     }
+    /**
+     * @description A bike as an edit carries it. `None` means "the reservation's period", which
+     *      is what a bike nobody had to arbitrate over keeps.
+     */
+    NewReservationBike: {
+      /** Format: date-time */
+      end_time?: string | null
+      /** Format: int32 */
+      id: number
+      /** Format: date-time */
+      start_time?: string | null
+    }
     /** @description The same choice, as the client sends it: only the id travels for a known unit. */
     NewReservationUnit:
       | {
@@ -906,7 +1016,7 @@ export interface components {
     }
     Reservation: {
       description: string
-      bikes: number[]
+      bikes: components['schemas']['ReservationBike'][]
       /** Format: date-time */
       end_time: string
       /** Format: int32 */
@@ -926,6 +1036,26 @@ export interface components {
       unit: components['schemas']['ReservationUnit']
       users: components['schemas']['UserSummary'][]
     }
+    /**
+     * @description One bike on a reservation, with the period it is actually held for.
+     *
+     *      Normally the reservation's own period. An override is how a conflict is
+     *      settled without moving the whole booking: the reservation keeps its hours
+     *      and only the disputed bike changes hands earlier.
+     */
+    ReservationBike: {
+      /** @description Whether that period is the bike's own rather than the reservation's */
+      custom: boolean
+      /** Format: date-time */
+      end_time: string
+      /** Format: int32 */
+      id: number
+      /**
+       * Format: date-time
+       * @description Already resolved: the bike's own period, or the reservation's
+       */
+      start_time: string
+    }
     /**
      * @description Only what the admin page lets somebody change. The unit, the requester, the
      *      telegram handle and the reason are shown but not editable, so they are not
@@ -933,7 +1063,11 @@ export interface components {
      *      rather than trusting a client to send them unchanged.
      */
     ReservationEditForm: {
-      bikes: number[]
+      /**
+       * @description Each with its own period when a conflict was settled by handing it over
+       *      early, and without one when it simply follows the reservation
+       */
+      bikes: components['schemas']['NewReservationBike'][]
       /** Format: date-time */
       end_time: string
       linka_emails: string[]

frontend/src/locales/en.yml

@@ -31,6 +31,7 @@ reservation:
   bikes-empty: No cargobike available for this period.
   bikes-error: Unable to load the cargobikes.
   bike-out-of-service: Out of service
+  bike-taken: Booked
   bike-size-large: Large cargo bikes
   bike-size-small: Small cargo bikes
   bike-size-empty: No bike of this size.
@@ -72,8 +73,26 @@ reservation:
     reason: Reason
     show-more: 'Show {n} more'
     show-less: Show less
+    own-period: This cargobike's own period
+  conflicts:
+    title: Cargobike already booked
+    intro: >-
+      These cargobikes are already held by another reservation over the same period.
+      You can save anyway — the call is yours.
+    held: 'held by {unit} (#{id}) from {from} to {to}'
+    give-back: 'Give the {bike} back on {at}'
+    take-later: 'Take the {bike} from {at}'
+    refused: One of the cargobikes is already booked over that period.
   edit:
     action: Edit
+    periods: Period per cargobike
+    follows: follows the reservation
+    own-period: Different period
+    follow-again: Follow the reservation
+    period-from: Taken from
+    period-to: Given back on
+    error-period: 'The period for the {bike} is empty or the wrong way round.'
+    error-period-outside: "The period for the {bike} must stay inside the reservation's."
     title: 'Reservation #{id}'
     intro: >-
       Only the period, the cargobikes and the Linka Go accounts can be changed.
@@ -162,6 +181,13 @@ admin:
       all-status: Any status
       count: No result | 1 reservation | {count} reservations
       empty: No reservation matches this search.
+    conflict:
+      title: 'Cannot approve: cargobike already booked'
+      line: 'The {bike} is already held by {unit} (#{id}), from {from} to {to}.'
+      hint: >-
+        Change that cargobike's period on either reservation, or drop it from this
+        one, then approve.
+      error: 'Approval refused: a cargobike is already booked over that period.'
     load-error: Unable to load the reservations.
     error: The status change failed.
     action:

frontend/src/locales/fr.yml

@@ -32,6 +32,7 @@ reservation:
   bikes-empty: Aucun cargobike disponible pour ce créneau.
   bikes-error: Impossible de charger les cargobikes.
   bike-out-of-service: Hors service
+  bike-taken: Réservé
   bike-size-large: Grands cargos
   bike-size-small: Petits cargos
   bike-size-empty: Aucun vélo de cette taille.
@@ -73,8 +74,26 @@ reservation:
     reason: Raison
     show-more: 'Afficher {n} de plus'
     show-less: Réduire
+    own-period: Période propre à ce cargobike
+  conflicts:
+    title: Cargobike déjà réservé
+    intro: >-
+      Ces cargobikes sont déjà tenus par une autre réservation sur la même plage.
+      Vous pouvez enregistrer quand même : à vous de trancher.
+    held: 'pris par {unit} (#{id}) du {from} au {to}'
+    give-back: 'Rendre le {bike} le {at}'
+    take-later: 'Prendre le {bike} à partir du {at}'
+    refused: Un des cargobikes est déjà réservé sur cette plage horaire.
   edit:
     action: Modifier
+    periods: Périodes par cargobike
+    follows: suit la réservation
+    own-period: Période différente
+    follow-again: Suivre la réservation
+    period-from: Pris à partir de
+    period-to: Rendu le
+    error-period: "La période du {bike} est vide ou à l'envers."
+    error-period-outside: 'La période du {bike} doit rester dans celle de la réservation.'
     title: 'Réservation #{id}'
     intro: >-
       Seuls la période, les cargobikes et les comptes Linka Go peuvent être modifiés.
@@ -163,6 +182,13 @@ admin:
       all-status: Tous les statuts
       count: Aucun résultat | 1 réservation | {count} réservations
       empty: Aucune réservation ne correspond à cette recherche.
+    conflict:
+      title: 'Validation impossible : cargobike déjà réservé'
+      line: 'Le {bike} est déjà pris par {unit} (#{id}), du {from} au {to}.'
+      hint: >-
+        Modifiez la période de ce cargobike sur l'une des deux réservations, ou
+        retirez-le de celle-ci, puis validez.
+      error: 'Validation refusée : un cargobike est déjà réservé sur cette plage.'
     load-error: Impossible de charger les réservations.
     error: Le changement de statut a échoué.
     action:

frontend/src/services/api/client.ts

@@ -3,7 +3,7 @@ import { HttpStatus } from 'http-status-ts'
 import type { QueryClient } from '@tanstack/vue-query'
 
 import type { paths } from '@/lib/api'
-import { Forbidden, Unauthorized } from '@/utils/types'
+import { ApiError, Forbidden, Unauthorized } from '@/utils/types'
 import { SESSION_KEY } from './keys'
 
 // Registered by main.ts. The interceptor below needs it to drop the session
@@ -41,7 +41,16 @@ client.use({
       } else if (response.status === HttpStatus.NOT_FOUND) {
         throw new Error(`Not found: ${response.url}`)
       }
-      throw new Error(
+      // The handlers answer with a plain string; keeping it lets a view show
+      // what actually went wrong instead of a generic failure. Cloned, so
+      // nothing downstream finds the body already read.
+      const detail = await response
+        .clone()
+        .text()
+        .catch(() => '')
+      throw new ApiError(
+        response.status,
+        detail.trim() ||
           `Unexpected error from ${response.url}: ${response.status} ${response.statusText}`,
       )
     }

frontend/src/services/api/reservations.ts

@@ -11,7 +11,14 @@ import { computed, toValue, type MaybeRefOrGetter } from 'vue'
 import { keepPreviousData, useMutation, useQuery, useQueryClient } from '@tanstack/vue-query'
 import { HttpStatus } from 'http-status-ts'
 
-import type { NewReservation, Reservation, ReservationEdit, ReservationStatus } from '@/utils/types'
+import type {
+  Conflict,
+  NewReservation,
+  NewReservationBike,
+  Reservation,
+  ReservationEdit,
+  ReservationStatus,
+} from '@/utils/types'
 import { getClient } from './client'
 
 export const RESERVATIONS_KEY = ['reservations']
@@ -70,6 +77,48 @@ export function useReservations(
   })
 }
 
+export const CONFLICTS_KEY = ['reservations', 'conflicts']
+
+/** The question a conflict check asks: a period, and the bikes wanted over it */
+export type ConflictProbe = {
+  /** The reservation being edited, so it does not conflict with itself */
+  reservation?: number
+  start_time: string
+  end_time: string
+  bikes: NewReservationBike[]
+}
+
+/**
+ * Which of the bikes wanted are already held by an approved or ongoing
+ * reservation over the same period.
+ *
+ * Overlap is a question about the whole table, so the database answers it: the
+ * browser never sees the bookings it is being compared against. A read behind a
+ * POST, because each bike may carry a period of its own — more than a query
+ * string can say.
+ */
+export function useConflicts(
+  probe: MaybeRefOrGetter<ConflictProbe | null>,
+  options: { enabled?: MaybeRefOrGetter<boolean> } = {},
+) {
+  const body = computed(() => toValue(probe))
+  return useQuery({
+    queryKey: computed(() => [...CONFLICTS_KEY, body.value]),
+    enabled: computed(() => (toValue(options.enabled) ?? true) && body.value !== null),
+    staleTime: 30 * 1000,
+    placeholderData: keepPreviousData,
+    queryFn: async () => {
+      const { data, response } = await getClient().POST('/api/reservations/conflicts', {
+        body: body.value!,
+      })
+      if (response.status !== HttpStatus.OK) {
+        throw new Error(`Unexpected status code received: ${response.status}`)
+      }
+      return (data ?? []) as Conflict[]
+    },
+  })
+}
+
 /**
  * The backend re-checks the state machine and answers 409 on a transition it
  * does not allow, so the buttons only have to offer the plausible ones.

frontend/src/utils/types.ts

@@ -18,6 +18,23 @@ export class Forbidden extends Error {
   }
 }
 
+/**
+ * Any other refusal, with the status and whatever the handler wrote. The
+ * handlers answer with a plain string, and some of them — a bike taken in the
+ * meantime, a period the backend rejects — are worth showing rather than
+ * flattening into "unexpected error".
+ */
+export class ApiError extends Error {
+  constructor(
+    readonly status: number,
+    message: string,
+  ) {
+    super(message)
+    this.name = 'ApiError'
+    Object.setPrototypeOf(this, ApiError.prototype)
+  }
+}
+
 // Shorthands over the generated schemas, so views never import `@/lib/api` directly
 export type Bike = components['schemas']['Bike']
 export type BikeStatus = components['schemas']['BikeStatus']
@@ -30,6 +47,9 @@ export type ReservationUnit = components['schemas']['ReservationUnit']
 export type NewReservation = components['schemas']['NewReservation']
 export type ReservationEdit = components['schemas']['ReservationEditForm']
 export type CalendarReservation = components['schemas']['CalendarReservation']
+export type ReservationBike = components['schemas']['ReservationBike']
+export type NewReservationBike = components['schemas']['NewReservationBike']
+export type Conflict = components['schemas']['Conflict']
 
 /**
  * What the details block can render: the fields the public calendar carries,

frontend/src/views/ReservationView.vue

@@ -4,6 +4,7 @@ import { useI18n } from 'vue-i18n'
 import { Plus, TriangleAlert, X } from '@lucide/vue'
 import { getLocalTimeZone, today, type DateValue } from '@internationalized/date'
 import { toast } from 'vue-sonner'
+import { HttpStatus } from 'http-status-ts'
 
 import DatePicker from '@/components/DatePicker.vue'
 import TelegramInput from '@/components/TelegramInput.vue'
@@ -16,9 +17,9 @@ import { Label } from '@/components/ui/label'
 import { Skeleton } from '@/components/ui/skeleton'
 import { Textarea } from '@/components/ui/textarea'
 import { useBikes } from '@/services/api/bikes'
-import { useCreateReservation } from '@/services/api/reservations'
+import { useConflicts, useCreateReservation } from '@/services/api/reservations'
 import { useSession } from '@/services/api/auth'
-import type { Bike, NewReservation } from '@/utils/types'
+import { ApiError, type Bike, type NewReservation } from '@/utils/types'
 
 const { t } = useI18n()
 
@@ -76,8 +77,30 @@ const periodPicked = computed(() => start.value !== null && end.value !== null)
 
 const { data: bikes, isPending: bikesPending, isError: bikesError } = useBikes()
 
+/**
+ * Which bikes an approved or ongoing reservation already holds over the period
+ * asked for. The overlap is worked out by the backend against the whole table:
+ * the browser is told "these are taken", not handed everybody's bookings to
+ * work it out itself.
+ */
+const probe = computed(() => {
+  if (!start.value || !end.value || end.value <= start.value) return null
+  return {
+    start_time: start.value.toISOString(),
+    end_time: end.value.toISOString(),
+    bikes: (bikes.value ?? []).map((bike) => ({ id: bike.id })),
+  }
+})
+const { data: conflicts } = useConflicts(probe)
+const taken = computed(() => new Set((conflicts.value ?? []).map((conflict) => conflict.bike)))
+
+function isTaken(bike: Bike) {
+  return taken.value.has(bike.id)
+}
+
+/** Out of service or already booked: either way it cannot be picked */
 function isUnavailable(bike: Bike) {
-  return bike.status === 'out_of_service'
+  return bike.status === 'out_of_service' || isTaken(bike)
 }
 
 const availableBikes = computed(() => (bikes.value ?? []).filter((bike) => !isUnavailable(bike)))
@@ -192,7 +215,12 @@ function submit() {
     },
     // The message is the backend's own: "bike 3 is out of service" is worth
     // reading, and a generic failure would hide it.
-    onError: (error) => toast.error(error.message || t('reservation.submit-error')),
+    onError: (error) =>
+      toast.error(
+        error instanceof ApiError && error.status === HttpStatus.CONFLICT
+          ? t('reservation.conflicts.refused')
+          : error.message || t('reservation.submit-error'),
+      ),
   })
 }
 </script>
@@ -348,7 +376,11 @@ function submit() {
                     v-if="isUnavailable(bike)"
                     class="text-destructive shrink-0 rounded px-1.5 py-0.5 text-[0.65rem] font-bold uppercase"
                   >
-                    {{ $t('reservation.bike-out-of-service') }}
+                    {{
+                      bike.status === 'out_of_service'
+                        ? $t('reservation.bike-out-of-service')
+                        : $t('reservation.bike-taken')
+                    }}
                   </span>
                 </button>
               </div>

src/api/reservations.rs

@@ -9,7 +9,7 @@
 use aide::{
     axum::{
         ApiRouter,
-        routing::{get_with, put_with},
+        routing::{get_with, post_with, put_with},
     },
     transform::TransformOperation,
 };
@@ -29,12 +29,10 @@ use crate::{
             AnonAppController, AppController, ControllerError,
             reservations::ReservationsControllerError,
         },
-        models::{
-            bike::BikeId,
-            reservation::{
-                CalendarReservation, NewReservation, Reservation, ReservationEdit, ReservationPage,
-                ReservationQuery, ReservationStatus,
-            },
+        models::reservation::{
+            CalendarReservation, Conflict, ConflictProbe, NewReservation, NewReservationBike,
+            Reservation, ReservationEdit, ReservationId, ReservationPage, ReservationQuery,
+            ReservationStatus,
         },
     },
 };
@@ -55,6 +53,7 @@ pub fn routes() -> ApiRouter {
             "/calendar",
             get_with(get_calendar_reservations, get_calendar_reservations_docs),
         )
+        .api_route("/conflicts", post_with(find_conflicts, find_conflicts_docs))
         .api_route(
             "/{id}",
             put_with(update_reservation, update_reservation_docs),
@@ -145,7 +144,8 @@ async fn create_reservation(
             err @ ReservationsControllerError::ReservationInvalid,
         )) => Err((StatusCode::BAD_REQUEST, err.to_string())),
         Err(ControllerError::Reservation(
-            err @ ReservationsControllerError::BikeOutOfService(_),
+            err @ (ReservationsControllerError::BikeOutOfService(_)
+            | ReservationsControllerError::BikesTaken(_)),
         )) => Err((StatusCode::CONFLICT, err.to_string())),
         Err(ControllerError::Reservation(
             err @ ReservationsControllerError::NotAMemberOfUnit(_),
@@ -169,7 +169,9 @@ fn create_reservation_docs(op: TransformOperation) -> TransformOperation {
         .response_with::<201, Json<Reservation>, _>(desc("The reservation, as stored"))
         .response_with::<400, (), _>(desc("The reservation is malformed"))
         .response_with::<403, (), _>(desc("The requester does not belong to the unit named"))
-        .response_with::<409, (), _>(desc("One of the bikes is out of service"))
+        .response_with::<409, (), _>(desc(
+            "One of the bikes is out of service, or is already held over that period",
+        ))
         .response_with::<422, (), _>(desc("The unit or one of the bikes does not exist"))
 }
 
@@ -229,15 +231,64 @@ fn get_my_reservations_docs(op: TransformOperation) -> TransformOperation {
         .response_with::<400, (), _>(desc("A status in the filter is not a known one"))
 }
 
-/// Only what the admin page lets somebody change. The unit, the requester, the
-/// telegram handle and the reason are shown but not editable, so they are not
-/// in the body at all: the handler reads them back from the stored reservation
-/// rather than trusting a client to send them unchanged.
+/// What a would-be booking asks about: a period, the bikes wanted, and the
+/// reservation to leave out of the answer when one is being edited.
+#[derive(Debug, Deserialize, JsonSchema)]
+struct ConflictProbeForm {
+    /// The reservation being edited, so it does not conflict with itself
+    reservation: Option<ReservationId>,
+    start_time: DateTime<Utc>,
+    end_time: DateTime<Utc>,
+    bikes: Vec<NewReservationBike>,
+}
+
+/// A read, not a write, but the question does not fit in a query string: each
+/// bike may carry a period of its own.
+#[axum::debug_handler]
+async fn find_conflicts(
+    ac: AppController,
+    Json(form): Json<ConflictProbeForm>,
+) -> Result<Json<Vec<Conflict>>, (StatusCode, String)> {
+    if form.end_time <= form.start_time {
+        return Err((StatusCode::BAD_REQUEST, "Empty period".to_owned()));
+    }
+    let probe = ConflictProbe {
+        reservation: form.reservation,
+        start_time: form.start_time,
+        end_time: form.end_time,
+        bikes: form.bikes,
+    };
+    match ac.find_conflicts(probe).await {
+        Ok(conflicts) => Ok(Json(conflicts)),
+        Err(err) => unexpected_error("find_conflicts", err),
+    }
+}
+
+fn find_conflicts_docs(op: TransformOperation) -> TransformOperation {
+    op.tag("Reservations")
+        .summary("Which of the bikes wanted are already held over the same period")
+        .description(
+            "Only approved and ongoing reservations hold a bike, so only they appear \
+             here. The booking form uses it to grey out what is taken, and the admin \
+             page to warn before double-booking on purpose.",
+        )
+        .response_with::<400, (), _>(desc("The period is empty or the wrong way round"))
+}
+
+/// Only what the admin page lets somebody change. The unit, the requester and
+/// the reason are shown but not editable, so they are not in the body at all:
+/// the handler reads them back from the stored reservation rather than trusting
+/// a client to send them unchanged.
 #[derive(Debug, Deserialize, JsonSchema)]
 struct ReservationEditForm {
     start_time: DateTime<Utc>,
     end_time: DateTime<Utc>,
-    bikes: Vec<BikeId>,
+    /// Whoever picks the bikes up may change, and with them the handle to
+    /// reach on the day
+    telegram: String,
+    /// Each with its own period when a conflict was settled by handing it over
+    /// early, and without one when it simply follows the reservation
+    bikes: Vec<NewReservationBike>,
     linka_emails: Vec<String>,
 }
 
@@ -261,7 +312,7 @@ async fn update_reservation(
         start_time: form.start_time,
         end_time: form.end_time,
         users: current.users.iter().map(|user| user.id).collect(),
-        telegram: current.telegram,
+        telegram: form.telegram,
         description: current.description,
         bikes: form.bikes,
         linka_emails: form.linka_emails,
@@ -302,7 +353,9 @@ fn update_reservation_docs(op: TransformOperation) -> TransformOperation {
         .response::<404, ()>()
         .response_with::<400, (), _>(desc("The reservation would become malformed"))
         .response_with::<409, (), _>(desc(
-            "A newly added bike is out of service, or the reservation is final",
+            "A newly added bike is out of service or already held over that period \
+             (managers may double-book on purpose, so this only reaches anybody else), \
+             or the reservation is final",
         ))
         .response_with::<422, (), _>(desc("One of the bikes does not exist"))
 }
@@ -333,7 +386,8 @@ async fn set_status(
     {
         Ok(()) => Ok(()),
         Err(ControllerError::Reservation(
-            err @ ReservationsControllerError::InvalidTransition(..),
+            err @ (ReservationsControllerError::InvalidTransition(..)
+            | ReservationsControllerError::BikesTaken(_)),
         )) => Err((StatusCode::CONFLICT, err.to_string())),
         Err(err) => unexpected_error("set_status", err),
     }
@@ -342,7 +396,11 @@ async fn set_status(
 fn set_status_docs(op: TransformOperation) -> TransformOperation {
     op.tag("Reservations")
         .summary("Move a reservation through its state machine")
-        .description("Refuses a transition the state machine does not allow, with a 409.")
+        .description(
+            "Refuses a transition the state machine does not allow, with a 409 — and, \
+             with the same status, approving a reservation whose bikes an approved \
+             one already holds over the same period.",
+        )
         .response_with::<403, (), _>(manager_desc)
         .response::<404, ()>()
         .response::<409, ()>()

src/core/controller/reservations.rs

@@ -1,17 +1,22 @@
 use chrono::{DateTime, Utc};
 use thiserror::Error;
 
-use crate::core::{
+use crate::{
+    core::{
         controller::{AnonAppController, AppController, ControllerError, ManagerAppController},
         models::{
-        bike::BikeStatus,
+            bike::{BikeId, BikeStatus},
             reservation::{
-            CalendarReservation, Involvement, NewReservation, NewReservationUnit, Reservation,
-            ReservationEdit, ReservationId, ReservationPage, ReservationQuery, ReservationStatus,
+                CalendarReservation, Conflict, ConflictProbe, Involvement, NewReservation,
+                NewReservationBike, NewReservationUnit, Reservation, ReservationBike,
+                ReservationEdit, ReservationId, ReservationPage, ReservationQuery,
+                ReservationStatus,
             },
             unit::UnitId,
         },
         repositories::RepositoryError,
+    },
+    services::telegram::{self, Notification},
 };
 
 /// Reading the reservations needs no session: the calendar is public.
@@ -114,10 +119,59 @@ impl AppController {
                 return Err(ReservationsControllerError::BikeOutOfService(*id).into());
             }
         }
-        self.db
+        // A bike held by an approved or ongoing reservation is not on offer.
+        // An admin is left free to double-book knowingly; the form they use
+        // warns them, and only they can sort the conflict out afterwards.
+        if !self.user().admin {
+            let conflicts = self
+                .db
+                .find_conflicts(ConflictProbe {
+                    reservation: None,
+                    start_time: reservation.start_time,
+                    end_time: reservation.end_time,
+                    bikes: reservation
+                        .bikes
+                        .iter()
+                        .map(|&id| NewReservationBike {
+                            id,
+                            start_time: None,
+                            end_time: None,
+                        })
+                        .collect(),
+                })
+                .await?;
+            if !conflicts.is_empty() {
+                return Err(ReservationsControllerError::BikesTaken(taken(&conflicts)).into());
+            }
+        }
+
+        let created = self
+            .db
             .create_reservation(reservation, self.user().id)
-            .await
-            .map_err(Into::into)
+            .await?;
+
+        // The group is told, in the background: a reservation is filed whether
+        // or not Telegram answered.
+        let mut names = Vec::with_capacity(created.bikes.len());
+        for held in &created.bikes {
+            names.push(match self.get_bike(held.id).await {
+                Ok(bike) => bike.name,
+                Err(_) => format!("#{}", held.id),
+            });
+        }
+        telegram::notify(Notification::reservation_requested(&created, names));
+
+        Ok(created)
+    }
+
+    /// Which of the bikes wanted are already held over the same period, and by
+    /// whom. Answering this needs a session but nothing more: it is what the
+    /// booking form greys out and what the admin page warns about.
+    pub async fn find_conflicts(
+        &self,
+        probe: ConflictProbe,
+    ) -> Result<Vec<Conflict>, ControllerError> {
+        self.db.find_conflicts(probe).await.map_err(Into::into)
     }
 }
 
@@ -159,7 +213,13 @@ impl AppController {
             if current.status != ReservationStatus::Requested
                 && (reservation.start_time != current.start_time
                     || reservation.end_time != current.end_time
-                    || !same_bikes(&reservation.bikes, &current.bikes))
+                    || reservation.telegram != current.telegram
+                    || !same_fleet(
+                        &reservation.bikes,
+                        &current.bikes,
+                        reservation.start_time,
+                        reservation.end_time,
+                    ))
             {
                 return Err(ReservationsControllerError::OnlyEmailsEditable(current.status).into());
             }
@@ -167,12 +227,31 @@ impl AppController {
 
         // A bike already on the reservation may well have broken down since:
         // only a newly added one has to be in service.
-        for id in &reservation.bikes {
-            if current.bikes.contains(id) {
+        for bike in &reservation.bikes {
+            if current.bikes.iter().any(|held| held.id == bike.id) {
                 continue;
             }
-            if self.get_bike(*id).await?.status == BikeStatus::OutOfService {
-                return Err(ReservationsControllerError::BikeOutOfService(*id).into());
+            if self.get_bike(bike.id).await?.status == BikeStatus::OutOfService {
+                return Err(ReservationsControllerError::BikeOutOfService(bike.id).into());
+            }
+        }
+
+        // Whoever manages the reservation may double-book on purpose — the
+        // admin page warns them and lets them decide. Anybody else may not:
+        // otherwise the rule the booking form applies would be one page refresh
+        // away from being bypassed.
+        if !manages {
+            let conflicts = self
+                .db
+                .find_conflicts(ConflictProbe {
+                    reservation: Some(reservation.id),
+                    start_time: reservation.start_time,
+                    end_time: reservation.end_time,
+                    bikes: reservation.bikes.clone(),
+                })
+                .await?;
+            if !conflicts.is_empty() {
+                return Err(ReservationsControllerError::BikesTaken(taken(&conflicts)).into());
             }
         }
 
@@ -195,10 +274,35 @@ impl AppController {
     }
 }
 
-/// Order carries no meaning here, so the two lists are compared as sets
-fn same_bikes(left: &[i32], right: &[i32]) -> bool {
-    let mut left = left.to_vec();
-    let mut right = right.to_vec();
+/// The bikes a set of conflicts is about, once each
+fn taken(conflicts: &[Conflict]) -> Vec<BikeId> {
+    let mut bikes: Vec<BikeId> = conflicts.iter().map(|conflict| conflict.bike).collect();
+    bikes.sort_unstable();
+    bikes.dedup();
+    bikes
+}
+
+/// Whether an edit leaves the fleet exactly as it is — the same bikes, each
+/// held over the same period. Order carries no meaning, so both are compared as
+/// sets, and the edit's periods are resolved first: leaving one out means "the
+/// reservation's own", which is what a bike nobody arbitrated over already has.
+fn same_fleet(
+    edit: &[NewReservationBike],
+    current: &[ReservationBike],
+    start: DateTime<Utc>,
+    end: DateTime<Utc>,
+) -> bool {
+    let mut left: Vec<_> = edit
+        .iter()
+        .map(|bike| {
+            let (from, to) = bike.period(start, end);
+            (bike.id, from, to)
+        })
+        .collect();
+    let mut right: Vec<_> = current
+        .iter()
+        .map(|bike| (bike.id, bike.start_time, bike.end_time))
+        .collect();
     left.sort_unstable();
     left.dedup();
     right.sort_unstable();
@@ -213,17 +317,46 @@ impl ManagerAppController {
         id: ReservationId,
         status: ReservationStatus,
     ) -> Result<(), ControllerError> {
-        let current = self.db.get_reservation(id).await?;
-        if current.unit.scope() != self.unit {
+        let reservation = self.db.get_reservation(id).await?;
+        if reservation.unit.scope() != self.unit {
             return Err(ControllerError::ImmutableUnitModificationError);
         }
-        let current = current.status;
+        let current = reservation.status;
         if current == status {
             return Ok(());
         }
         if !current.can_transition_to(status) {
             return Err(ReservationsControllerError::InvalidTransition(current, status).into());
         }
+
+        // Approving is the moment a reservation really takes its bikes, so it is
+        // the moment a double booking stops being a warning and becomes a
+        // refusal: two approved reservations over one bike means somebody turns
+        // up to an empty rack. Only this transition is guarded — a reservation
+        // already approved is allowed to start, whatever was overridden earlier.
+        if status == ReservationStatus::Approved {
+            let conflicts = self
+                .db
+                .find_conflicts(ConflictProbe {
+                    reservation: Some(id),
+                    start_time: reservation.start_time,
+                    end_time: reservation.end_time,
+                    bikes: reservation
+                        .bikes
+                        .iter()
+                        .map(|held| NewReservationBike {
+                            id: held.id,
+                            start_time: Some(held.start_time),
+                            end_time: Some(held.end_time),
+                        })
+                        .collect(),
+                })
+                .await?;
+            if !conflicts.is_empty() {
+                return Err(ReservationsControllerError::BikesTaken(taken(&conflicts)).into());
+            }
+        }
+
         self.db
             .set_reservation_status(id, status)
             .await
@@ -254,20 +387,105 @@ pub enum ReservationsControllerError {
     NotOnTheReservation,
     #[error("A reservation that is {0:?} only accepts a change of Linka Go accounts")]
     OnlyEmailsEditable(ReservationStatus),
+    #[error("Cargobike(s) {0:?} are already booked over that period")]
+    BikesTaken(Vec<BikeId>),
 }
 
 #[cfg(test)]
 mod tests {
-    use super::same_bikes;
+    use chrono::{TimeZone, Utc};
+
+    use super::{same_fleet, taken};
+    use crate::core::models::reservation::{
+        Conflict, NewReservationBike, ReservationBike, ReservationStatus,
+    };
+
+    fn at(hour: u32) -> chrono::DateTime<Utc> {
+        Utc.with_ymd_and_hms(2026, 8, 25, hour, 0, 0).unwrap()
+    }
+
+    /// A bike on the edit, following the reservation unless told otherwise
+    fn wanted(id: i32, period: Option<(u32, u32)>) -> NewReservationBike {
+        NewReservationBike {
+            id,
+            start_time: period.map(|(from, _)| at(from)),
+            end_time: period.map(|(_, to)| at(to)),
+        }
+    }
+
+    fn held(id: i32, from: u32, to: u32) -> ReservationBike {
+        ReservationBike {
+            id,
+            start_time: at(from),
+            end_time: at(to),
+            custom: false,
+        }
+    }
 
     #[test]
-    fn bike_lists_compare_as_sets() {
-        assert!(same_bikes(&[1, 2], &[2, 1]), "order carries no meaning");
-        assert!(same_bikes(&[1, 1, 2], &[2, 1]), "nor do repeats");
-        assert!(same_bikes(&[], &[]));
+    fn fleets_compare_as_sets_of_bikes_and_periods() {
+        let (start, end) = (at(12), at(20));
+        let current = [held(1, 12, 20), held(2, 12, 20)];
 
-        assert!(!same_bikes(&[1, 2], &[1]), "one was removed");
-        assert!(!same_bikes(&[1], &[1, 2]), "one was added");
-        assert!(!same_bikes(&[1], &[2]));
+        assert!(
+            same_fleet(&[wanted(2, None), wanted(1, None)], &current, start, end),
+            "order carries no meaning"
+        );
+        assert!(
+            same_fleet(
+                &[wanted(1, None), wanted(1, None), wanted(2, None)],
+                &current,
+                start,
+                end
+            ),
+            "nor do repeats"
+        );
+        assert!(
+            same_fleet(
+                &[wanted(1, Some((12, 20))), wanted(2, None)],
+                &current,
+                start,
+                end
+            ),
+            "spelling out the reservation's own period changes nothing"
+        );
+
+        assert!(
+            !same_fleet(&[wanted(1, None)], &current, start, end),
+            "one was removed"
+        );
+        assert!(
+            !same_fleet(
+                &[wanted(1, None), wanted(2, None), wanted(3, None)],
+                &current,
+                start,
+                end
+            ),
+            "one was added"
+        );
+        assert!(
+            !same_fleet(
+                &[wanted(1, Some((12, 15))), wanted(2, None)],
+                &current,
+                start,
+                end
+            ),
+            "one is handed over early"
+        );
+    }
+
+    #[test]
+    fn conflicting_bikes_are_named_once() {
+        let conflict = |bike| Conflict {
+            bike,
+            reservation: 1,
+            unit: "PolyNite".to_owned(),
+            status: ReservationStatus::Approved,
+            start_time: at(12),
+            end_time: at(20),
+        };
+        // The same bike can be held by two reservations at once
+        assert_eq!(taken(&[conflict(2), conflict(1), conflict(2)]), vec![1, 2]);
+        assert!(taken(&[]).is_empty());
     }
 }

src/core/models/reservation.rs

@@ -30,6 +30,9 @@ impl ReservationStatus {
             (self, next),
             (Requested, Refused)
                 | (Requested, Approved)
+                // Back to the queue: an admin who approved too quickly, or who
+                // needs the bikes freed while the booking is discussed
+                | (Approved, Requested)
                 | (Approved, Cancelled)
                 | (Approved, Ongoing)
                 | (Ongoing, Cancelled)
@@ -139,6 +142,79 @@ impl NewReservationUnit {
     }
 }
 
+/// One bike on a reservation, with the period it is actually held for.
+///
+/// Normally the reservation's own period. An override is how a conflict is
+/// settled without moving the whole booking: the reservation keeps its hours
+/// and only the disputed bike changes hands earlier.
+#[derive(Debug, Serialize, Deserialize, Clone, JsonSchema, PartialEq, Eq)]
+pub struct ReservationBike {
+    pub id: BikeId,
+    /// Already resolved: the bike's own period, or the reservation's
+    pub start_time: DateTime<Utc>,
+    pub end_time: DateTime<Utc>,
+    /// Whether that period is the bike's own rather than the reservation's
+    pub custom: bool,
+}
+
+/// A bike as an edit carries it. `None` means "the reservation's period", which
+/// is what a bike nobody had to arbitrate over keeps.
+#[derive(Debug, Serialize, Deserialize, Clone, JsonSchema, PartialEq, Eq)]
+pub struct NewReservationBike {
+    pub id: BikeId,
+    pub start_time: Option<DateTime<Utc>>,
+    pub end_time: Option<DateTime<Utc>>,
+}
+
+impl NewReservationBike {
+    /// The period this bike is held for, given the reservation's own
+    pub fn period(
+        &self,
+        start: DateTime<Utc>,
+        end: DateTime<Utc>,
+    ) -> (DateTime<Utc>, DateTime<Utc>) {
+        (
+            self.start_time.unwrap_or(start),
+            self.end_time.unwrap_or(end),
+        )
+    }
+
+    /// Both bounds or neither, the right way round, and inside the
+    /// reservation's own period: a bike cannot be held when nothing is booked.
+    fn is_valid(&self, start: DateTime<Utc>, end: DateTime<Utc>) -> bool {
+        match (self.start_time, self.end_time) {
+            (None, None) => true,
+            (Some(from), Some(to)) => from < to && from >= start && to <= end,
+            _ => false,
+        }
+    }
+}
+
+/// A bike wanted over a period, as a conflict check asks about it.
+#[derive(Debug, Clone, PartialEq, Eq)]
+pub struct ConflictProbe {
+    /// The reservation being edited, left out of its own check
+    pub reservation: Option<ReservationId>,
+    pub start_time: DateTime<Utc>,
+    pub end_time: DateTime<Utc>,
+    pub bikes: Vec<NewReservationBike>,
+}
+
+/// A bike the probe wants that somebody else already holds over part of the
+/// same period. Only the reservations that actually hold a bike — approved and
+/// ongoing — can produce one.
+#[derive(Debug, Serialize, Deserialize, Clone, JsonSchema, PartialEq, Eq)]
+pub struct Conflict {
+    pub bike: BikeId,
+    pub reservation: ReservationId,
+    /// Who holds it, to be named in the warning
+    pub unit: String,
+    pub status: ReservationStatus,
+    /// When the bike is held, which is what a resolution has to step around
+    pub start_time: DateTime<Utc>,
+    pub end_time: DateTime<Utc>,
+}
+
 #[derive(Debug, Serialize, Deserialize, Clone, JsonSchema, PartialEq, Eq)]
 pub struct Reservation {
     pub id: ReservationId,
@@ -150,7 +226,7 @@ pub struct Reservation {
     pub users: Vec<UserSummary>,
     pub telegram: String,
     pub description: String,
-    pub bikes: Vec<BikeId>,
+    pub bikes: Vec<ReservationBike>,
     /// Linka Go accounts allowed to unlock the bikes. Plain addresses: they
     /// need not belong to anybody who ever logs into this app.
     pub linka_emails: Vec<String>,
@@ -167,7 +243,8 @@ pub struct CalendarReservation {
     pub unit: ReservationUnit,
     pub start_time: DateTime<Utc>,
     pub end_time: DateTime<Utc>,
-    pub bikes: Vec<BikeId>,
+    /// With their own periods: a bike handed over early is drawn as such
+    pub bikes: Vec<ReservationBike>,
     pub status: ReservationStatus,
 }
 
@@ -282,7 +359,9 @@ pub struct ReservationEdit {
     pub users: Vec<UserId>,
     pub telegram: String,
     pub description: String,
-    pub bikes: Vec<BikeId>,
+    /// Each with the period it is held for, or nothing to follow the
+    /// reservation's own
+    pub bikes: Vec<NewReservationBike>,
     pub linka_emails: Vec<String>,
 }
 
@@ -322,6 +401,10 @@ impl ReservationEdit {
             && self.end_time > self.start_time
             && !self.bikes.is_empty()
             && !self.users.is_empty()
+            && self
+                .bikes
+                .iter()
+                .all(|bike| bike.is_valid(self.start_time, self.end_time))
             && telegram_valid(&self.telegram)
             && linka_emails_valid(&self.linka_emails)
     }

src/core/repositories/reservations_repository.rs

@@ -3,8 +3,8 @@ use async_trait::async_trait;
 use crate::core::{
     models::{
         reservation::{
-            NewReservation, Reservation, ReservationEdit, ReservationId, ReservationPage,
-            ReservationQuery, ReservationStatus,
+            Conflict, ConflictProbe, NewReservation, Reservation, ReservationEdit, ReservationId,
+            ReservationPage, ReservationQuery, ReservationStatus,
         },
         unit::UnitId,
         user::UserId,
@@ -30,6 +30,11 @@ pub trait ReservationsRepository {
     ) -> Result<Vec<Reservation>, RepositoryError>;
     async fn get_reservation(&self, id: ReservationId) -> Result<Reservation, RepositoryError>;
 
+    /// The bikes in `probe` that somebody else already holds over part of the
+    /// same period, one row per overlap. Only approved and ongoing reservations
+    /// hold a bike, and a reservation is never in conflict with itself.
+    async fn find_conflicts(&self, probe: ConflictProbe) -> Result<Vec<Conflict>, RepositoryError>;
+
     /// Always stored as `Requested`: the state machine starts here. The
     /// requester comes from the session, not from the request body.
     async fn create_reservation(

src/services/database/reservations.rs

@@ -12,8 +12,9 @@ use crate::{
     core::{
         models::{
             reservation::{
-                NewReservation, NewReservationUnit, Reservation, ReservationEdit, ReservationId,
-                ReservationPage, ReservationQuery, ReservationStatus, ReservationUnit,
+                Conflict, ConflictProbe, NewReservation, NewReservationBike, NewReservationUnit,
+                Reservation, ReservationEdit, ReservationId, ReservationPage, ReservationQuery,
+                ReservationStatus, ReservationUnit,
             },
             unit::{Unit, UnitId},
             user::UserId,
@@ -73,7 +74,7 @@ struct ReservationDB {
     pub status: ReservationStatusDB,
     pub linka_emails: Vec<String>,
     pub users: Value,
-    pub bikes: Vec<i32>,
+    pub bikes: Value,
 }
 
 impl TryFrom<ReservationDB> for Reservation {
@@ -100,13 +101,35 @@ impl TryFrom<ReservationDB> for Reservation {
             users: serde_json::from_value(value.users)?,
             telegram: value.telegram,
             description: value.description,
-            bikes: value.bikes,
+            bikes: serde_json::from_value(value.bikes)?,
             linka_emails: value.linka_emails,
             status: value.status.into(),
         })
     }
 }
 
+struct ConflictDB {
+    pub bike: i32,
+    pub reservation: i32,
+    pub unit: String,
+    pub status: ReservationStatusDB,
+    pub start_time: DateTime<Utc>,
+    pub end_time: DateTime<Utc>,
+}
+
+impl From<ConflictDB> for Conflict {
+    fn from(value: ConflictDB) -> Self {
+        Conflict {
+            bike: value.bike,
+            reservation: value.reservation,
+            unit: value.unit,
+            status: value.status.into(),
+            start_time: value.start_time,
+            end_time: value.end_time,
+        }
+    }
+}
+
 /// The page query carries one extra column — how many rows the filter matches
 /// in all, from a window function — so it needs its own row type. Everything
 /// else is the shared one, which the conversion below hands back.
@@ -123,7 +146,7 @@ struct ReservationPageRowDB {
     pub status: ReservationStatusDB,
     pub linka_emails: Vec<String>,
     pub users: Value,
-    pub bikes: Vec<i32>,
+    pub bikes: Value,
     pub total: i64,
 }
 
@@ -152,7 +175,7 @@ impl SqlxDatabase {
         tx: &mut sqlx::Transaction<'_, sqlx::Postgres>,
         id: ReservationId,
         users: &[i32],
-        bikes: &[i32],
+        bikes: &[NewReservationBike],
     ) -> Result<(), RepositoryError> {
         query!(
             r#"DELETE FROM reservations_users WHERE reservation_id = $1"#,
@@ -175,11 +198,18 @@ impl SqlxDatabase {
         )
         .execute(&mut **tx)
         .await?;
+        // Three parallel arrays rather than a row constructor: it keeps the
+        // insert to one statement, and NULL still means "the reservation's period"
+        let bike_ids: Vec<i32> = bikes.iter().map(|bike| bike.id).collect();
+        let starts: Vec<Option<DateTime<Utc>>> = bikes.iter().map(|bike| bike.start_time).collect();
+        let ends: Vec<Option<DateTime<Utc>>> = bikes.iter().map(|bike| bike.end_time).collect();
         query!(
-            r#"INSERT INTO reservations_bikes (reservation_id, bike_id)
-                SELECT $1, UNNEST($2::integer[])"#,
+            r#"INSERT INTO reservations_bikes (reservation_id, bike_id, start_time, end_time)
+                SELECT $1, * FROM UNNEST($2::integer[], $3::timestamptz[], $4::timestamptz[])"#,
             id,
-            bikes
+            &bike_ids,
+            &starts as &[Option<DateTime<Utc>>],
+            &ends as &[Option<DateTime<Utc>>]
         )
         .execute(&mut **tx)
         .await?;
@@ -266,10 +296,19 @@ impl ReservationsRepository for SqlxDatabase {
                     JOIN users u ON u.id = ru.user_id
                     WHERE ru.reservation_id = r.id
                 ), '[]'::json) AS "users!",
-                ARRAY(
-                    SELECT bike_id FROM reservations_bikes
-                    WHERE reservation_id = r.id ORDER BY bike_id
-                ) AS "bikes!",
+                -- Each bike with the period it is actually held for: its own
+                -- when a conflict was settled by handing it over early, the
+                -- reservation's otherwise
+                COALESCE((
+                    SELECT json_agg(json_build_object(
+                        'id', rb.bike_id,
+                        'start_time', COALESCE(rb.start_time, r.start_time),
+                        'end_time', COALESCE(rb.end_time, r.end_time),
+                        'custom', rb.start_time IS NOT NULL
+                    ) ORDER BY rb.bike_id)
+                    FROM reservations_bikes rb
+                    WHERE rb.reservation_id = r.id
+                ), '[]'::json) AS "bikes!",
                 -- The size of the whole match, alongside the page itself: the
                 -- caller can show "42 results" without a second round trip
                 COUNT(*) OVER () AS "total!"
@@ -365,10 +404,19 @@ impl ReservationsRepository for SqlxDatabase {
                     JOIN users u ON u.id = ru.user_id
                     WHERE ru.reservation_id = r.id
                 ), '[]'::json) AS "users!",
-                ARRAY(
-                    SELECT bike_id FROM reservations_bikes
-                    WHERE reservation_id = r.id ORDER BY bike_id
-                ) AS "bikes!"
+                -- Each bike with the period it is actually held for: its own
+                -- when a conflict was settled by handing it over early, the
+                -- reservation's otherwise
+                COALESCE((
+                    SELECT json_agg(json_build_object(
+                        'id', rb.bike_id,
+                        'start_time', COALESCE(rb.start_time, r.start_time),
+                        'end_time', COALESCE(rb.end_time, r.end_time),
+                        'custom', rb.start_time IS NOT NULL
+                    ) ORDER BY rb.bike_id)
+                    FROM reservations_bikes rb
+                    WHERE rb.reservation_id = r.id
+                ), '[]'::json) AS "bikes!"
             FROM reservations r
             LEFT JOIN units un ON un.id = r.unit_id
             WHERE r.unit_id = $1
@@ -382,6 +430,56 @@ impl ReservationsRepository for SqlxDatabase {
         .collect::<Result<Vec<_>, _>>()?)
     }
 
+    async fn find_conflicts(&self, probe: ConflictProbe) -> Result<Vec<Conflict>, RepositoryError> {
+        // The probe resolved: every bike with the period it would be held for
+        let mut bikes = Vec::with_capacity(probe.bikes.len());
+        let mut starts = Vec::with_capacity(probe.bikes.len());
+        let mut ends = Vec::with_capacity(probe.bikes.len());
+        for bike in &probe.bikes {
+            let (from, to) = bike.period(probe.start_time, probe.end_time);
+            bikes.push(bike.id);
+            starts.push(from);
+            ends.push(to);
+        }
+
+        Ok(query_as!(
+            ConflictDB,
+            r#"WITH wanted(bike_id, start_time, end_time) AS (
+                SELECT * FROM UNNEST($1::integer[], $2::timestamptz[], $3::timestamptz[])
+            )
+            SELECT
+                w.bike_id AS "bike!",
+                r.id AS "reservation!",
+                COALESCE(un."name", r.unit_label, '') AS "unit!",
+                r.status AS "status: ReservationStatusDB",
+                COALESCE(rb.start_time, r.start_time) AS "start_time!",
+                COALESCE(rb.end_time, r.end_time) AS "end_time!"
+            FROM wanted w
+            JOIN reservations_bikes rb ON rb.bike_id = w.bike_id
+            JOIN reservations r ON r.id = rb.reservation_id
+            LEFT JOIN units un ON un.id = r.unit_id
+            -- Only a reservation that actually holds the bike can conflict: a
+            -- request holds nothing yet, and a final one holds nothing any more
+            WHERE r.status IN ('approved', 'ongoing')
+              -- A reservation never conflicts with itself
+              AND ($4::integer IS NULL OR r.id <> $4)
+              -- Half-open overlap: ending exactly when the other starts is fine,
+              -- which is what makes handing a bike over at 15:00 a resolution
+              AND COALESCE(rb.start_time, r.start_time) < w.end_time
+              AND COALESCE(rb.end_time, r.end_time) > w.start_time
+            ORDER BY w.bike_id, "start_time!""#,
+            &bikes,
+            &starts,
+            &ends,
+            probe.reservation,
+        )
+        .fetch_all(&self.pool)
+        .await?
+        .into_iter()
+        .map(Into::into)
+        .collect())
+    }
+
     async fn get_reservation(&self, id: ReservationId) -> Result<Reservation, RepositoryError> {
         Ok(query_as!(
             ReservationDB,
@@ -410,10 +508,19 @@ impl ReservationsRepository for SqlxDatabase {
                     JOIN users u ON u.id = ru.user_id
                     WHERE ru.reservation_id = r.id
                 ), '[]'::json) AS "users!",
-                ARRAY(
-                    SELECT bike_id FROM reservations_bikes
-                    WHERE reservation_id = r.id ORDER BY bike_id
-                ) AS "bikes!"
+                -- Each bike with the period it is actually held for: its own
+                -- when a conflict was settled by handing it over early, the
+                -- reservation's otherwise
+                COALESCE((
+                    SELECT json_agg(json_build_object(
+                        'id', rb.bike_id,
+                        'start_time', COALESCE(rb.start_time, r.start_time),
+                        'end_time', COALESCE(rb.end_time, r.end_time),
+                        'custom', rb.start_time IS NOT NULL
+                    ) ORDER BY rb.bike_id)
+                    FROM reservations_bikes rb
+                    WHERE rb.reservation_id = r.id
+                ), '[]'::json) AS "bikes!"
             FROM reservations r
             LEFT JOIN units un ON un.id = r.unit_id
             WHERE r.id = $1"#,
@@ -459,7 +566,18 @@ impl ReservationsRepository for SqlxDatabase {
         if !users.contains(&requester) {
             users.push(requester);
         }
-        Self::set_reservation_links(&mut tx, id, &users, &reservation.bikes).await?;
+        // A new reservation holds every bike for its own period; only an edit
+        // can hand one over early.
+        let bikes: Vec<NewReservationBike> = reservation
+            .bikes
+            .iter()
+            .map(|&id| NewReservationBike {
+                id,
+                start_time: None,
+                end_time: None,
+            })
+            .collect();
+        Self::set_reservation_links(&mut tx, id, &users, &bikes).await?;
 
         tx.commit().await?;
 

src/services/mod.rs

@@ -1,2 +1,3 @@
 //! External services used by the core: database, and any third party api you add.
 pub mod database;
+pub mod telegram;

src/services/telegram.rs

@@ -0,0 +1,233 @@
+//! Telegram notifications to the group that runs the cargobikes.
+//!
+//! The point of this module is that adding a message is a two-line change:
+//! a variant on [`Notification`] and an arm in [`Notification::render`].
+//! Nothing else in the app has to know that Telegram exists, nor how a message
+//! is worded — a caller says *what happened*, not *what to write*.
+//!
+//! Sending never blocks the request that triggered it and never fails it: a
+//! reservation is filed whether or not the group was told about it. When the
+//! bot is not configured, [`notify`] is a no-op, which is what a development
+//! machine and the test suite want.
+
+use std::sync::OnceLock;
+
+use chrono::{DateTime, Utc};
+use openidconnect::reqwest;
+use serde::Serialize;
+use tracing::{debug, error, warn};
+
+use crate::{core::models::reservation::Reservation, utils::config};
+
+/// Something worth telling the group about.
+///
+/// Each variant carries what the message needs, already resolved: the renderer
+/// reads no database and can therefore never fail nor be slow.
+#[derive(Debug, Clone)]
+pub enum Notification {
+    /// A reservation request has just been filed
+    ReservationRequested {
+        id: i32,
+        unit: String,
+        requester: String,
+        start_time: DateTime<Utc>,
+        end_time: DateTime<Utc>,
+        bikes: Vec<String>,
+        description: String,
+    },
+}
+
+impl Notification {
+    /// The message body, in Telegram's HTML parse mode. Only `&`, `<` and `>`
+    /// need escaping there, which [`escape`] does for every value that comes
+    /// from a user.
+    fn render(&self) -> String {
+        match self {
+            Notification::ReservationRequested {
+                id,
+                unit,
+                requester,
+                start_time,
+                end_time,
+                bikes,
+                description,
+            } => {
+                let bikes = if bikes.is_empty() {
+                    "—".to_owned()
+                } else {
+                    escape(&bikes.join(", "))
+                };
+                format!(
+                    "🚲 <b>Nouvelle demande de réservation #{id}</b>\n\
+                     Association : {unit}\n\
+                     Demandée par : {requester}\n\
+                     Période : {start} → {end}\n\
+                     Cargobike(s) : {bikes}\n\
+                     Raison : {description}",
+                    unit = escape(unit),
+                    requester = escape(requester),
+                    start = format_moment(*start_time),
+                    end = format_moment(*end_time),
+                    description = escape(description),
+                )
+            }
+        }
+    }
+}
+
+impl Notification {
+    /// The notification a freshly filed request produces, given the reservation
+    /// as stored and the names of the bikes it holds.
+    pub fn reservation_requested(reservation: &Reservation, bikes: Vec<String>) -> Self {
+        Notification::ReservationRequested {
+            id: reservation.id,
+            unit: reservation.unit.label().to_owned(),
+            requester: reservation
+                .users
+                .iter()
+                .find(|user| user.id == reservation.requester)
+                .map_or_else(
+                    || format!("#{}", reservation.requester),
+                    |user| format!("{} {}", user.firstname, user.name),
+                ),
+            start_time: reservation.start_time,
+            end_time: reservation.end_time,
+            bikes,
+            description: reservation.description.clone(),
+        }
+    }
+}
+
+/// Sends `notification` to the configured group, in the background.
+///
+/// Returns immediately. A failure is logged and goes no further: the caller's
+/// own work has already succeeded, and undoing it because Telegram is down
+/// would be worse than a missing message.
+pub fn notify(notification: Notification) {
+    let Some(telegram) = config::get().telegram.as_ref() else {
+        debug!("[TELEGRAM] not configured, dropping {notification:?}");
+        return;
+    };
+    let token = telegram.bot_token.clone();
+    let chat_id = telegram.chat_id.clone();
+    let api_url = telegram.get_api_url().to_owned();
+
+    tokio::spawn(async move {
+        if let Err(err) = send(&api_url, &token, &chat_id, &notification.render()).await {
+            error!("[TELEGRAM] could not send the notification: {err}");
+        }
+    });
+}
+
+#[derive(Serialize)]
+struct SendMessage<'a> {
+    chat_id: &'a str,
+    text: &'a str,
+    parse_mode: &'a str,
+    /// Link previews turn a reservation into a wall of nothing
+    disable_web_page_preview: bool,
+}
+
+async fn send(api_url: &str, token: &str, chat_id: &str, text: &str) -> Result<(), String> {
+    // Serialised by hand: the http client is the one `openidconnect` brings, and
+    // it is built without its `json` feature.
+    let body = serde_json::to_string(&SendMessage {
+        chat_id,
+        text,
+        parse_mode: "HTML",
+        disable_web_page_preview: true,
+    })
+    .map_err(|err| err.to_string())?;
+
+    let response = http_client()
+        .post(format!("{api_url}/bot{token}/sendMessage"))
+        .header("content-type", "application/json")
+        .body(body)
+        .send()
+        .await
+        .map_err(|err| err.to_string())?;
+
+    if !response.status().is_success() {
+        // Telegram explains itself in the body, and that is the only way to
+        // tell "wrong token" from "the bot is not in that group"
+        let status = response.status();
+        let body = response.text().await.unwrap_or_default();
+        warn!("[TELEGRAM] {status}: {body}");
+        return Err(format!("{status}: {body}"));
+    }
+    Ok(())
+}
+
+fn http_client() -> &'static reqwest::Client {
+    static HTTP_CLIENT: OnceLock<reqwest::Client> = OnceLock::new();
+    HTTP_CLIENT.get_or_init(|| {
+        reqwest::ClientBuilder::new()
+            .timeout(std::time::Duration::from_secs(10))
+            .build()
+            .expect("Unable to build the telegram http client")
+    })
+}
+
+/// The three characters Telegram's HTML mode reads as markup
+fn escape(value: &str) -> String {
+    value
+        .replace('&', "&amp;")
+        .replace('<', "&lt;")
+        .replace('>', "&gt;")
+}
+
+/// Swiss local time, which is the only one the group cares about
+fn format_moment(moment: DateTime<Utc>) -> String {
+    moment
+        .with_timezone(&chrono::FixedOffset::east_opt(2 * 3600).expect("valid offset"))
+        .format("%d.%m.%Y %H:%M")
+        .to_string()
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    fn moment(day: u32, hour: u32) -> DateTime<Utc> {
+        use chrono::TimeZone;
+        Utc.with_ymd_and_hms(2026, 8, day, hour, 0, 0).unwrap()
+    }
+
+    #[test]
+    fn a_request_reads_as_a_message() {
+        let message = Notification::ReservationRequested {
+            id: 12,
+            unit: "PolyNite".to_owned(),
+            requester: "Milan Hyenne".to_owned(),
+            start_time: moment(25, 10),
+            end_time: moment(26, 16),
+            bikes: vec!["1000".to_owned(), "2000".to_owned()],
+            description: "Transport du matériel".to_owned(),
+        }
+        .render();
+
+        assert!(message.contains("#12"));
+        assert!(message.contains("PolyNite"));
+        assert!(message.contains("1000, 2000"));
+        // Rendered in local time: 10:00 UTC is noon in Lausanne
+        assert!(message.contains("25.08.2026 12:00"), "{message}");
+    }
+
+    #[test]
+    fn markup_in_a_user_value_is_escaped() {
+        let message = Notification::ReservationRequested {
+            id: 1,
+            unit: "<b>Fake</b> & Co".to_owned(),
+            requester: "A".to_owned(),
+            start_time: moment(25, 10),
+            end_time: moment(25, 12),
+            bikes: vec![],
+            description: String::new(),
+        }
+        .render();
+
+        assert!(message.contains("&lt;b&gt;Fake&lt;/b&gt; &amp; Co"));
+        // The heading is ours, and stays markup
+        assert!(message.contains("<b>Nouvelle demande"));
+    }
+}

src/utils/config.rs

@@ -31,6 +31,26 @@ pub struct OidcConfig {
     pub session_lifetime: Option<i64>,
 }
 
+/// The bot that posts to the group. Absent, nothing is sent — which is what a
+/// development machine wants. Both values are secrets: keep them in `.env` or
+/// in the environment, not in a committed file.
+#[derive(Deserialize, Clone)]
+pub struct TelegramConfig {
+    pub bot_token: String,
+    pub chat_id: String,
+    /// Where the bot api lives. Only ever set to something else to point the
+    /// sender at a stub, or at a proxy in a network that cannot reach Telegram.
+    pub api_url: Option<String>,
+}
+
+impl TelegramConfig {
+    pub fn get_api_url(&self) -> &str {
+        self.api_url
+            .as_deref()
+            .unwrap_or("https://api.telegram.org")
+    }
+}
+
 /// A user that can be logged in without going through the provider.
 /// Only usable in debug builds, see `POST /api/login`.
 #[derive(Deserialize, Clone)]
@@ -50,6 +70,9 @@ pub struct AppConfig {
     pub server: ServerConfig,
     pub postgres: PostgresConfig,
     pub oidc: OidcConfig,
+    /// Absent when the bot is not set up: notifications are then dropped
+    #[serde(default)]
+    pub telegram: Option<TelegramConfig>,
     #[serde(default)]
     pub dev_users: Vec<DevUserConfig>,
     /// Directory containing the built frontend
@@ -89,11 +112,18 @@ impl AppConfig {
 
 /// Loads the configuration, by decreasing priority:
 ///  - `APP__SERVER__PORT=3000` style environment variables
+///  - the same variables read from `./.env`, which is where the secrets live in
+///    development (a real environment variable still wins over the file)
 ///  - `./config.yml`
 ///  - the file pointed by `$APP_CONFIG` (`/etc/cargagep/config.yml` by default)
 pub fn get() -> &'static AppConfig {
     static CONFIG: OnceLock<AppConfig> = OnceLock::new();
     CONFIG.get_or_init(|| {
+        // `.env` is read into the environment first, so `APP__*` written there
+        // behaves exactly like a variable exported by hand. Anything already in
+        // the environment is left alone.
+        load_dotenv();
+
         let config_path =
             std::env::var("APP_CONFIG").unwrap_or("/etc/cargagep/config.yml".to_owned());
         let config_path = Path::new(&config_path);
@@ -112,3 +142,27 @@ pub fn get() -> &'static AppConfig {
             .expect("Invalid config format")
     })
 }
+
+/// A minimal `.env` reader: `KEY=value` per line, `#` comments, optional
+/// surrounding quotes. Deliberately not a dependency — the file is ours, and
+/// dbmate already reads it with the same rules.
+fn load_dotenv() {
+    let Ok(contents) = std::fs::read_to_string(".env") else {
+        return;
+    };
+    for line in contents.lines() {
+        let line = line.trim();
+        if line.is_empty() || line.starts_with('#') {
+            continue;
+        }
+        let Some((key, value)) = line.split_once('=') else {
+            continue;
+        };
+        let key = key.trim().trim_start_matches("export ").trim();
+        let value = value.trim().trim_matches('"').trim_matches('\'');
+        // An exported variable wins: that is how a deployment overrides the file
+        if std::env::var_os(key).is_none() {
+            unsafe { std::env::set_var(key, value) };
+        }
+    }
+}