diff --git a/backend/src/index.ts b/backend/src/index.ts
index 3fde079..c336ba5 100644
--- a/backend/src/index.ts
+++ b/backend/src/index.ts
@@ -132,17 +132,17 @@ async function setAppSetting(key: string, value: string) {
   );
 }
 
-async function getOrganizationById(
+async function getOrganizationByZammadId(
   client: { query: (text: string, params?: unknown[]) => Promise<{ rows: OrganizationRow[]; rowCount: number | null }> },
-  organizationId: number
+  zammadId: number
 ) {
   const result = await client.query(
     `
-      SELECT id, zammad_id, name, synced_at
+      SELECT zammad_id::text AS id, zammad_id::text AS zammad_id, name, synced_at
       FROM organizations
-      WHERE id = $1;
+      WHERE zammad_id = $1;
     `,
-    [organizationId]
+    [zammadId]
   );
 
   return result.rows[0] ?? null;
@@ -332,7 +332,7 @@ async function ensureRecurringSessionsForUserPeriod(userId: string, startIso: st
         rb.valid_until::text,
         rb.start_time::text
       FROM recurring_billings rb
-      JOIN organizations o ON o.id = rb.organization_id
+      JOIN organizations o ON o.zammad_id = rb.organization_id
       WHERE rb.user_id = $1
         AND rb.valid_from < $3::date
         AND (rb.valid_until IS NULL OR rb.valid_until >= $2::date);
@@ -558,7 +558,7 @@ async function getPeriodOverview(config: PeriodConfig, period: ParsedPeriod, use
           NULL::timestamptz AS closed_at
         FROM tickets t
         JOIN sessions s ON s.ticket_id = t.id
-        LEFT JOIN organizations o ON o.id = t.organization_id
+        LEFT JOIN organizations o ON o.zammad_id = t.organization_id
         WHERE s.started_at >= $1::timestamptz AND s.started_at < $2::timestamptz
           AND s.user_id = $3
         GROUP BY
@@ -587,7 +587,7 @@ async function getPeriodOverview(config: PeriodConfig, period: ParsedPeriod, use
           s.rounded_minutes
         FROM sessions s
         JOIN tickets t ON t.id = s.ticket_id
-        LEFT JOIN organizations so ON so.id = s.organization_id
+        LEFT JOIN organizations so ON so.zammad_id = s.organization_id
         WHERE s.started_at >= $1::timestamptz
           AND s.started_at < $2::timestamptz
           AND s.user_id = $3
@@ -681,8 +681,8 @@ async function getStatisticsOverview(period: ParsedPeriod, userId: string) {
         s.recurring_billing_id
       FROM sessions s
       JOIN tickets t ON t.id = s.ticket_id
-      LEFT JOIN organizations so ON so.id = s.organization_id
-      LEFT JOIN organizations ot ON ot.id = t.organization_id
+      LEFT JOIN organizations so ON so.zammad_id = s.organization_id
+      LEFT JOIN organizations ot ON ot.zammad_id = t.organization_id
       WHERE s.started_at >= $1::timestamptz
         AND s.started_at < $2::timestamptz
         AND s.user_id = $3
@@ -904,8 +904,8 @@ async function getStatisticsOverview(period: ParsedPeriod, userId: string) {
           s.rounded_minutes
         FROM sessions s
         JOIN tickets t ON t.id = s.ticket_id
-        LEFT JOIN organizations so ON so.id = s.organization_id
-        LEFT JOIN organizations o ON o.id = t.organization_id
+        LEFT JOIN organizations so ON so.zammad_id = s.organization_id
+        LEFT JOIN organizations o ON o.zammad_id = t.organization_id
         WHERE s.started_at >= $1::timestamptz
           AND s.started_at < $2::timestamptz
           AND s.user_id = $3
@@ -1066,7 +1066,7 @@ async function getPeriodTicket(config: PeriodConfig, period: ParsedPeriod, ticke
         COALESCE(period_sessions.open_count, 0)::int AS ticket_open_count,
         COALESCE(period_sessions.manual_session_count, 0)::int AS ticket_manual_session_count
       FROM tickets t
-      LEFT JOIN organizations o ON o.id = t.organization_id
+      LEFT JOIN organizations o ON o.zammad_id = t.organization_id
       ${periodClosureJoin}
       LEFT JOIN LATERAL (
         SELECT
@@ -1096,13 +1096,13 @@ async function getPeriodTicket(config: PeriodConfig, period: ParsedPeriod, ticke
         (
           SELECT name
           FROM organizations
-          WHERE id = sessions.organization_id
+          WHERE zammad_id = sessions.organization_id
         ) AS organization_name,
         COALESCE(
           (
             SELECT name
             FROM organizations
-            WHERE id = sessions.organization_id
+            WHERE zammad_id = sessions.organization_id
           ),
           customer_name
         ) AS customer_name,
@@ -1473,7 +1473,7 @@ app.get("/api/organizations", async (req, res) => {
 
   const result = await query<OrganizationRow>(
     `
-      SELECT id, zammad_id, name, synced_at
+      SELECT zammad_id::text AS id, zammad_id::text AS zammad_id, name, synced_at
       FROM organizations
       ${where.length > 0 ? `WHERE ${where.join(" AND ")}` : ""}
       ORDER BY lower(name) ASC
@@ -1596,30 +1596,30 @@ app.post("/api/admin/zammad/organizations/sync", requireAdmin, async (req, res)
     {
       const staleResult =
         zammadIds.length > 0
-          ? await client.query<{ id: string }>(
+          ? await client.query<{ zammad_id: string }>(
               `
-                SELECT id
+                SELECT zammad_id::text AS zammad_id
                 FROM organizations
                 WHERE NOT (zammad_id = ANY($1::bigint[]));
               `,
               [zammadIds]
             )
-          : await client.query<{ id: string }>(
+          : await client.query<{ zammad_id: string }>(
               `
-                SELECT id
+                SELECT zammad_id::text AS zammad_id
                 FROM organizations;
               `
             );
-      const staleIds = staleResult.rows.map((row) => row.id);
+      const staleZammadIds = staleResult.rows.map((row) => row.zammad_id);
 
-      if (staleIds.length > 0) {
+      if (staleZammadIds.length > 0) {
         const ticketResult = await client.query(
           `
             UPDATE tickets
             SET organization_id = NULL
             WHERE organization_id = ANY($1::bigint[]);
           `,
-          [staleIds]
+          [staleZammadIds]
         );
         const sessionResult = await client.query(
           `
@@ -1627,14 +1627,14 @@ app.post("/api/admin/zammad/organizations/sync", requireAdmin, async (req, res)
             SET organization_id = NULL
             WHERE organization_id = ANY($1::bigint[]);
           `,
-          [staleIds]
+          [staleZammadIds]
         );
         const deleteResult = await client.query(
           `
             DELETE FROM organizations
-            WHERE id = ANY($1::bigint[]);
+            WHERE zammad_id = ANY($1::bigint[]);
           `,
-          [staleIds]
+          [staleZammadIds]
         );
 
         unlinkedTickets = ticketResult.rowCount ?? 0;
@@ -1980,8 +1980,8 @@ app.get("/api/export/months/:month", requireUser, async (req, res) => {
         s.activity
       FROM sessions s
       JOIN tickets t ON t.id = s.ticket_id
-      LEFT JOIN organizations so ON so.id = s.organization_id
-      LEFT JOIN organizations ot ON ot.id = t.organization_id
+      LEFT JOIN organizations so ON so.zammad_id = s.organization_id
+      LEFT JOIN organizations ot ON ot.zammad_id = t.organization_id
       LEFT JOIN ticket_day_billings tdb
         ON tdb.ticket_id = s.ticket_id
        AND tdb.user_id = s.user_id
@@ -2149,7 +2149,7 @@ async function recurringBillingResponse(userId: string) {
         ) AS slots
       FROM recurring_billings rb
       JOIN users owner ON owner.id = rb.user_id
-      JOIN organizations o ON o.id = rb.organization_id
+      JOIN organizations o ON o.zammad_id = rb.organization_id
       LEFT JOIN recurring_billing_slots rbs ON rbs.recurring_billing_id = rb.id
       WHERE rb.user_id = $1
       GROUP BY rb.id, owner.username, owner.display_name, o.name
@@ -2170,7 +2170,7 @@ app.post("/api/recurring-billings", requireUser, async (req, res) => {
   const payload = parseRecurringBillingPayload(req.body);
 
   const created = await withTransaction(async (client) => {
-    const organization = await getOrganizationById(client, payload.organizationId);
+    const organization = await getOrganizationByZammadId(client, payload.organizationId);
 
     if (!organization) {
       return "organization-not-found" as const;
@@ -2272,9 +2272,9 @@ app.patch("/api/recurring-billings/:billingId", requireUser, async (req, res) =>
       `
         SELECT id
         FROM recurring_billings
-        WHERE s.id = $1
-          AND s.user_id = $2
-        FOR UPDATE OF s;
+        WHERE id = $1
+          AND user_id = $2
+        FOR UPDATE;
       `,
       [billingId, userId]
     );
@@ -2283,7 +2283,7 @@ app.patch("/api/recurring-billings/:billingId", requireUser, async (req, res) =>
       return { status: "not-found" as const };
     }
 
-    const organization = await getOrganizationById(client, payload.organizationId);
+    const organization = await getOrganizationByZammadId(client, payload.organizationId);
 
     if (!organization) {
       return { status: "organization-not-found" as const };
@@ -2579,7 +2579,7 @@ app.post("/api/sessions", requireUser, async (req, res) => {
       throw badRequest("organizationId is required");
     }
 
-    const organization = await getOrganizationById(client, effectiveOrganizationId);
+    const organization = await getOrganizationByZammadId(client, effectiveOrganizationId);
 
     if (!organization) {
       throw badRequest("organizationId must reference an existing organization");
@@ -2670,7 +2670,7 @@ app.get("/api/tickets/lookup", requireUser, async (req, res) => {
         COALESCE(o.name, t.customer_name) AS customer_name,
         t.work_type
       FROM tickets t
-      LEFT JOIN organizations o ON o.id = t.organization_id
+      LEFT JOIN organizations o ON o.zammad_id = t.organization_id
       WHERE t.ticket_number = $1;
     `,
     [ticketNumber]
@@ -2709,7 +2709,7 @@ app.get("/api/tickets/search", requireUser, async (req, res) => {
         COALESCE(SUM(s.rounded_minutes), 0)::int AS total_minutes
       FROM tickets t
       JOIN sessions s ON s.ticket_id = t.id
-      LEFT JOIN organizations o ON o.id = t.organization_id
+      LEFT JOIN organizations o ON o.zammad_id = t.organization_id
       WHERE s.user_id = $1
         AND (
           t.ticket_number ILIKE $2
@@ -2745,7 +2745,7 @@ app.patch("/api/tickets/:ticketId", requireUser, async (req, res) => {
 
   try {
     const result = await withTransaction(async (client) => {
-      const organization = await getOrganizationById(client, organizationId);
+      const organization = await getOrganizationByZammadId(client, organizationId);
 
       if (!organization) {
         return "organization-not-found" as const;
@@ -3232,7 +3232,7 @@ app.patch("/api/sessions/:sessionId/details", requireUser, async (req, res) => {
   const roundedMinutes = Math.max(1, Math.round(durationSeconds / 60));
 
   const updated = await withTransaction(async (client) => {
-    const organization = await getOrganizationById(client, organizationId);
+    const organization = await getOrganizationByZammadId(client, organizationId);
 
     if (!organization) {
       return "organization-not-found" as const;
diff --git a/backend/src/migrations.ts b/backend/src/migrations.ts
index 99d1c0f..8c49f9d 100644
--- a/backend/src/migrations.ts
+++ b/backend/src/migrations.ts
@@ -97,7 +97,7 @@ export async function migrate() {
     CREATE TABLE IF NOT EXISTS tickets (
       id BIGSERIAL PRIMARY KEY,
       ticket_number TEXT NOT NULL UNIQUE,
-      organization_id BIGINT REFERENCES organizations(id),
+      organization_id BIGINT REFERENCES organizations(zammad_id),
       customer_name TEXT,
       work_type TEXT,
       created_at TIMESTAMPTZ NOT NULL DEFAULT now(),
@@ -107,7 +107,7 @@ export async function migrate() {
   `);
 
   await query("ALTER TABLE tickets ADD COLUMN IF NOT EXISTS customer_name TEXT;");
-  await query("ALTER TABLE tickets ADD COLUMN IF NOT EXISTS organization_id BIGINT REFERENCES organizations(id);");
+  await query("ALTER TABLE tickets ADD COLUMN IF NOT EXISTS organization_id BIGINT REFERENCES organizations(zammad_id);");
   await query("ALTER TABLE tickets ADD COLUMN IF NOT EXISTS work_type TEXT;");
   await query(`
     ALTER TABLE tickets
@@ -130,7 +130,7 @@ export async function migrate() {
     CREATE TABLE IF NOT EXISTS sessions (
       id BIGSERIAL PRIMARY KEY,
       ticket_id BIGINT NOT NULL REFERENCES tickets(id) ON DELETE CASCADE,
-      organization_id BIGINT REFERENCES organizations(id),
+      organization_id BIGINT REFERENCES organizations(zammad_id),
       customer_name TEXT NOT NULL,
       activity TEXT NOT NULL,
       work_type TEXT NOT NULL,
@@ -151,7 +151,7 @@ export async function migrate() {
   `);
 
   await query("ALTER TABLE sessions ADD COLUMN IF NOT EXISTS user_id BIGINT REFERENCES users(id);");
-  await query("ALTER TABLE sessions ADD COLUMN IF NOT EXISTS organization_id BIGINT REFERENCES organizations(id);");
+  await query("ALTER TABLE sessions ADD COLUMN IF NOT EXISTS organization_id BIGINT REFERENCES organizations(zammad_id);");
   await query("ALTER TABLE sessions ADD COLUMN IF NOT EXISTS recurring_billing_id BIGINT;");
   await query("ALTER TABLE sessions ADD COLUMN IF NOT EXISTS recurring_billing_slot_id BIGINT;");
   await query("ALTER TABLE sessions ADD COLUMN IF NOT EXISTS recurring_occurrence_date DATE;");
@@ -173,7 +173,7 @@ export async function migrate() {
       id BIGSERIAL PRIMARY KEY,
       user_id BIGINT NOT NULL REFERENCES users(id) ON DELETE CASCADE,
       ticket_number TEXT,
-      organization_id BIGINT NOT NULL REFERENCES organizations(id),
+      organization_id BIGINT NOT NULL REFERENCES organizations(zammad_id),
       activity TEXT NOT NULL,
       work_type TEXT NOT NULL,
       recurrence_type TEXT NOT NULL,
@@ -191,6 +191,82 @@ export async function migrate() {
     );
   `);
 
+  await query(`
+    ALTER TABLE tickets DROP CONSTRAINT IF EXISTS tickets_organization_id_fkey;
+    ALTER TABLE sessions DROP CONSTRAINT IF EXISTS sessions_organization_id_fkey;
+    ALTER TABLE recurring_billings DROP CONSTRAINT IF EXISTS recurring_billings_organization_id_fkey;
+  `);
+
+  await query(`
+    UPDATE tickets t
+    SET organization_id = o.zammad_id
+    FROM organizations o
+    WHERE t.organization_id = o.id
+      AND NOT EXISTS (
+        SELECT 1
+        FROM organizations current_org
+        WHERE current_org.zammad_id = t.organization_id
+      );
+  `);
+
+  await query(`
+    UPDATE sessions s
+    SET organization_id = o.zammad_id
+    FROM organizations o
+    WHERE s.organization_id = o.id
+      AND NOT EXISTS (
+        SELECT 1
+        FROM organizations current_org
+        WHERE current_org.zammad_id = s.organization_id
+      );
+  `);
+
+  await query(`
+    UPDATE recurring_billings rb
+    SET organization_id = o.zammad_id
+    FROM organizations o
+    WHERE rb.organization_id = o.id
+      AND NOT EXISTS (
+        SELECT 1
+        FROM organizations current_org
+        WHERE current_org.zammad_id = rb.organization_id
+      );
+  `);
+
+  await query(`
+    UPDATE tickets
+    SET organization_id = NULL
+    WHERE organization_id IS NOT NULL
+      AND NOT EXISTS (
+        SELECT 1
+        FROM organizations o
+        WHERE o.zammad_id = tickets.organization_id
+      );
+  `);
+
+  await query(`
+    UPDATE sessions
+    SET organization_id = NULL
+    WHERE organization_id IS NOT NULL
+      AND NOT EXISTS (
+        SELECT 1
+        FROM organizations o
+        WHERE o.zammad_id = sessions.organization_id
+      );
+  `);
+
+  await query(`
+    ALTER TABLE tickets
+      ADD CONSTRAINT tickets_organization_id_fkey
+      FOREIGN KEY (organization_id) REFERENCES organizations(zammad_id);
+    ALTER TABLE sessions
+      ADD CONSTRAINT sessions_organization_id_fkey
+      FOREIGN KEY (organization_id) REFERENCES organizations(zammad_id);
+    ALTER TABLE recurring_billings
+      ADD CONSTRAINT recurring_billings_organization_id_fkey
+      FOREIGN KEY (organization_id) REFERENCES organizations(zammad_id);
+  `);
+
   await query("ALTER TABLE recurring_billings ALTER COLUMN ticket_number DROP NOT NULL;");
   await query("ALTER TABLE recurring_billings DROP CONSTRAINT IF EXISTS recurring_billings_ticket_number_format;");
   await query("UPDATE recurring_billings SET recurrence_type = 'every_n_weeks' WHERE recurrence_type = 'every_n_days';");