docs: def downgrade():
      """Downgrade database."""
      # table ObjectVersionTag
      op.drop_table('files_objecttags')
      # table ObjectVersion: modify primary_key
      if op.get_context().dialect.name == 'mysql':
          op.execute(
              'ALTER TABLE files_object '
              'DROP INDEX uq_files_object_bucket_id, '
              'DROP PRIMARY KEY, '
              'ADD PRIMARY KEY(`bucket_id`, `key`, `version_id`)')
      else:
          op.drop_constraint(
              'pk_files_object', 'files_object', type_='primary')
          op.create_primary_key('pk_files_object', 'files_object',
                                ['bucket_id', 'key', 'version_id'])