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  # Copyright 2013 OpenStack Foundation
  #
  # Licensed under the Apache License, Version 2.0 (the "License"); you may
  # not use this file except in compliance with the License. You may obtain
  # a copy of the License at
  #
  #      http://www.apache.org/licenses/LICENSE-2.0
  #
  # Unless required by applicable law or agreed to in writing, software
  # distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
  # WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the
  # License for the specific language governing permissions and limitations
  # under the License.
  
  import sqlalchemy
  
  from keystone.common.sql import migration_helpers
  
  
  def list_constraints(migrate_engine):
      meta = sqlalchemy.MetaData()
      meta.bind = migrate_engine
      user_table = sqlalchemy.Table('user', meta, autoload=True)
      proj_table = sqlalchemy.Table('project', meta, autoload=True)
      cred_table = sqlalchemy.Table('credential', meta, autoload=True)
  
      constraints = [{'table': cred_table,
                      'fk_column': 'user_id',
                      'ref_column': user_table.c.id},
                     {'table': cred_table,
                      'fk_column': 'project_id',
                      'ref_column': proj_table.c.id}]
      return constraints
  
  
  def upgrade(migrate_engine):
      # SQLite does not support constraints, and querying the constraints
      # raises an exception
      if migrate_engine.name == 'sqlite':
          return
      migration_helpers.remove_constraints(list_constraints(migrate_engine))
  
  
  def downgrade(migrate_engine):
      if migrate_engine.name == 'sqlite':
          return
      migration_helpers.add_constraints(list_constraints(migrate_engine))
  