flask-migrate cannot drop table because other objects depend on it

Question

Problem applying just created migration (Added db.Model) through Flask-Migrate (SQLAlchemy) for PostgresSQL DB. The error itself:

sqlalchemy.exc.InternalError: (psycopg2.InternalError) cannot drop table parameter_subtype because other objects depend on it

Full error stack trace is:

INFO  [alembic.autogenerate.compare] Detected removed foreign key (event_id) (id) on table stage_has_event
INFO  [alembic.autogenerate.compare] Detected removed column 'stage_has_event.event_id'
  Generating /app/migrations/versions/e224df1a4818_.py ... done
(venv) $ ./manage db upgrade
INFO  [alembic.runtime.migration] Context impl PostgresqlImpl.
INFO  [alembic.runtime.migration] Will assume transactional DDL.
INFO  [alembic.runtime.migration] Running upgrade cadf22871ae0 -> e224df1a4818, empty message
Traceback (most recent call last):
  File "/app/venv/bin/flask", line 11, in <module>
    sys.exit(main())
  File "/app/venv/lib/python2.7/site-packages/flask_cli/cli.py", line 502, in main
    cli.main(args=args, prog_name=name)
  File "/app/venv/lib/python2.7/site-packages/flask_cli/cli.py", line 369, in main
    return AppGroup.main(self, *args, **kwargs)
  File "/app/venv/lib/python2.7/site-packages/click/core.py", line 697, in main
    rv = self.invoke(ctx)
  File "/app/venv/lib/python2.7/site-packages/click/core.py", line 1066, in invoke
    return _process_result(sub_ctx.command.invoke(sub_ctx))
  File "/app/venv/lib/python2.7/site-packages/click/core.py", line 1066, in invoke
    return _process_result(sub_ctx.command.invoke(sub_ctx))
  File "/app/venv/lib/python2.7/site-packages/click/core.py", line 895, in invoke
    return ctx.invoke(self.callback, **ctx.params)
  File "/app/venv/lib/python2.7/site-packages/click/core.py", line 535, in invoke
    return callback(*args, **kwargs)
  File "/app/venv/lib/python2.7/site-packages/click/decorators.py", line 17, in new_func
    return f(get_current_context(), *args, **kwargs)
  File "/app/venv/lib/python2.7/site-packages/flask/cli.py", line 257, in decorator
    return __ctx.invoke(f, *args, **kwargs)
  File "/app/venv/lib/python2.7/site-packages/click/core.py", line 535, in invoke
    return callback(*args, **kwargs)
  File "/app/venv/lib/python2.7/site-packages/flask_migrate/cli.py", line 134, in upgrade
    _upgrade(directory, revision, sql, tag, x_arg)
  File "/app/venv/lib/python2.7/site-packages/flask_migrate/__init__.py", line 259, in upgrade
    command.upgrade(config, revision, sql=sql, tag=tag)
  File "/app/venv/lib/python2.7/site-packages/alembic/command.py", line 254, in upgrade
    script.run_env()
  File "/app/venv/lib/python2.7/site-packages/alembic/script/base.py", line 427, in run_env
    util.load_python_file(self.dir, 'env.py')
  File "/app/venv/lib/python2.7/site-packages/alembic/util/pyfiles.py", line 81, in load_python_file
    module = load_module_py(module_id, path)
  File "/app/venv/lib/python2.7/site-packages/alembic/util/compat.py", line 141, in load_module_py
    mod = imp.load_source(module_id, path, fp)
  File "migrations/env.py", line 87, in <module>
    run_migrations_online()
  File "migrations/env.py", line 80, in run_migrations_online
    context.run_migrations()
  File "<string>", line 8, in run_migrations
  File "/app/venv/lib/python2.7/site-packages/alembic/runtime/environment.py", line 836, in run_migrations
    self.get_context().run_migrations(**kw)
  File "/app/venv/lib/python2.7/site-packages/alembic/runtime/migration.py", line 330, in run_migrations
    step.migration_fn(**kw)
  File "/app/migrations/versions/e224df1a4818_.py", line 21, in upgrade
    op.drop_table('parameter_subtype')
  File "<string>", line 8, in drop_table
  File "<string>", line 3, in drop_table
  File "/app/venv/lib/python2.7/site-packages/alembic/operations/ops.py", line 1187, in drop_table
    operations.invoke(op)
  File "/app/venv/lib/python2.7/site-packages/alembic/operations/base.py", line 319, in invoke
    return fn(self, operation)
  File "/app/venv/lib/python2.7/site-packages/alembic/operations/toimpl.py", line 70, in drop_table
    operation.to_table(operations.migration_context)
  File "/app/venv/lib/python2.7/site-packages/alembic/ddl/impl.py", line 203, in drop_table
    self._exec(schema.DropTable(table))
  File "/app/venv/lib/python2.7/site-packages/alembic/ddl/impl.py", line 118, in _exec
    return conn.execute(construct, *multiparams, **params)
  File "/app/venv/lib/python2.7/site-packages/sqlalchemy/engine/base.py", line 948, in execute
    return meth(self, multiparams, params)
  File "/app/venv/lib/python2.7/site-packages/sqlalchemy/sql/ddl.py", line 68, in _execute_on_connection
    return connection._execute_ddl(self, multiparams, params)
  File "/app/venv/lib/python2.7/site-packages/sqlalchemy/engine/base.py", line 1009, in _execute_ddl
    compiled
  File "/app/venv/lib/python2.7/site-packages/sqlalchemy/engine/base.py", line 1200, in _execute_context
    context)
  File "/app/venv/lib/python2.7/site-packages/sqlalchemy/engine/base.py", line 1413, in _handle_dbapi_exception
    exc_info
  File "/app/venv/lib/python2.7/site-packages/sqlalchemy/util/compat.py", line 203, in raise_from_cause
    reraise(type(exception), exception, tb=exc_tb, cause=cause)
  File "/app/venv/lib/python2.7/site-packages/sqlalchemy/engine/base.py", line 1193, in _execute_context
    context)
  File "/app`enter code here`/venv/lib/python2.7/site-packages/sqlalchemy/engine/default.py", line 507, in do_execute
    cursor.execute(statement, parameters)
sqlalchemy.exc.InternalError: (psycopg2.InternalError) cannot drop table parameter_subtype because other objects depend on it
DETAIL:  constraint parameter_parameter_subtype_id_fkey on table parameter depends on table parameter_subtype
HINT:  Use DROP ... CASCADE to drop the dependent objects too.
 [SQL: '\nDROP TABLE parameter_subtype']

This leads to table not created in PostgreSQL db and columns either. Unsure what may have caused this problem and how to understand this Error message.

I did manually drop the tables and it helped. However how to be with migration. Any Suggestions?

Answer 1

Flask-Migrate does not read in the database to see the dependencies between the objects. You can achieve a successful migration by reordering the drop_table in your migration file.

What you can do is modify the migration file like so:

op.drop_table("parameter_subtype")
op.drop_table("parameter")

And then run the upgrade command. Normally ordering the drop in this order should solve the problem.