Why this change is needed:
PostgreSQLDB class doesn't have a fetch() method. The migration code
was incorrectly using db.fetch() for batch data retrieval, causing
AttributeError during E2E tests.
How it solves it:
1. Changed db.fetch(sql, params) to db.query(sql, params, multirows=True)
2. Updated all test mocks to support the multirows parameter
3. Consolidated mock_query implementation to handle both single and multi-row queries
Impact:
- PostgreSQL legacy data migration now works correctly in E2E tests
- All unit tests pass (6/6)
- Aligns with PostgreSQLDB's actual API
Testing:
- pytest tests/test_postgres_migration.py -v (6/6 passed)
- Updated test_postgres_migration_trigger mock
- Updated test_scenario_2_legacy_upgrade_migration mock
- Updated base mock_pg_db fixture
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