migrations/1781000000006_reconcile-user-cards-unique.js

@@ -64,17 +64,25 @@ export const up = (pgm) => {
   // Re-assert the canonical 3-col UNIQUE from `initial-schema.js`.
   // No-op against initial-schema-applied envs (the constraint already
   // exists under its auto-generated name); constructive against any
-  // env where the constraint was somehow dropped. Wrapped in
+  // env where the constraint was somehow dropped. Postgres does not
-  // `DO $$ ... EXCEPTION WHEN duplicate_object` because Postgres
+  // support `ADD CONSTRAINT ... IF NOT EXISTS`, so we pre-check
-  // does not support `ADD CONSTRAINT ... IF NOT EXISTS`.
+  // `pg_constraint`. (An earlier draft used `EXCEPTION WHEN
+  // duplicate_object`, but `ADD CONSTRAINT UNIQUE` raises
+  // SQLSTATE 42P07 `duplicate_table` from the auto-created
+  // supporting index, not 42710 `duplicate_object` — the pre-check
+  // sidesteps both.)
   pgm.sql(`
     DO $$
     BEGIN
+      IF NOT EXISTS (
+        SELECT 1 FROM pg_constraint
+        WHERE conname = 'user_cards_user_id_card_id_is_foil_key'
+          AND conrelid = 'user_cards'::regclass
+      ) THEN
         ALTER TABLE user_cards
         ADD CONSTRAINT user_cards_user_id_card_id_is_foil_key
         UNIQUE (user_id, card_id, is_foil);
-    EXCEPTION
+      END IF;
-      WHEN duplicate_object THEN NULL;
     END $$;
   `);
 };