diff --git a/hindsight-api-slim/hindsight_api/alembic/versions/f6g7h8i9j0k1_chunk_fk_cascade_delete.py b/hindsight-api-slim/hindsight_api/alembic/versions/f6g7h8i9j0k1_chunk_fk_cascade_delete.py
index f0311496..96031733 100644
--- a/hindsight-api-slim/hindsight_api/alembic/versions/f6g7h8i9j0k1_chunk_fk_cascade_delete.py
+++ b/hindsight-api-slim/hindsight_api/alembic/versions/f6g7h8i9j0k1_chunk_fk_cascade_delete.py
@@ -24,9 +24,28 @@ def upgrade() -> None:
     the memory_units rows survived with chunk_id = NULL, leaving ghost records.
     Switching to CASCADE ensures they are removed together with their chunk.
     """
-    op.drop_constraint("memory_units_chunk_fkey", "memory_units", type_="foreignkey")
-    op.create_foreign_key(
-        "memory_units_chunk_fkey", "memory_units", "chunks", ["chunk_id"], ["chunk_id"], ondelete="CASCADE"
+    from alembic import context
+
+    schema = context.config.get_main_option("target_schema")
+    schema_prefix = f'"{schema}".' if schema else ""
+    # Use raw SQL with IF EXISTS so this is safe on schemas where the FK was
+    # already dropped or never existed under this name.
+    op.execute(f"ALTER TABLE {schema_prefix}memory_units DROP CONSTRAINT IF EXISTS memory_units_chunk_fkey")
+    # Use a DO block so the ADD is also idempotent: if the FK already exists (e.g.
+    # the schema was provisioned after the base migration already added it) the
+    # duplicate_object exception is swallowed rather than failing the migration.
+    op.execute(
+        f"""
+        DO $$ BEGIN
+            ALTER TABLE {schema_prefix}memory_units
+                ADD CONSTRAINT memory_units_chunk_fkey
+                FOREIGN KEY (chunk_id)
+                REFERENCES {schema_prefix}chunks (chunk_id)
+                ON DELETE CASCADE;
+        EXCEPTION
+            WHEN duplicate_object THEN NULL;
+        END $$;
+        """
     )