diff --git a/apps/portal-bff/prisma/migrations/20260526143000_add_org_hierarchy/migration.sql b/apps/portal-bff/prisma/migrations/20260526143000_add_org_hierarchy/migration.sql
index 49c5cc2..ffe4372 100644
--- a/apps/portal-bff/prisma/migrations/20260526143000_add_org_hierarchy/migration.sql
+++ b/apps/portal-bff/prisma/migrations/20260526143000_add_org_hierarchy/migration.sql
@@ -83,9 +83,13 @@ ALTER TABLE "delegations" ADD CONSTRAINT "delegations_region_code_fkey"
     ON DELETE RESTRICT ON UPDATE CASCADE;
 
 -- AddForeignKey
+-- ON DELETE SET NULL because delegationCode is optional in the Prisma
+-- schema (Structure.delegation Delegation?). Matches Prisma's default
+-- for nullable relations; the drift gate would otherwise flag this
+-- on every `prisma migrate dev`.
 ALTER TABLE "structures" ADD CONSTRAINT "structures_delegation_code_fkey"
     FOREIGN KEY ("delegation_code") REFERENCES "delegations"("code")
-    ON DELETE RESTRICT ON UPDATE CASCADE;
+    ON DELETE SET NULL ON UPDATE CASCADE;
 
 -- ------------------------------------------------------------------
 -- Test-tenant reference seed (per ADR-0027 §"Seeding posture").