Downgrade database.

def downgrade():
    """Downgrade database."""
    op.drop_constraint(op.f('fk_oauth2server_token_user_id_accounts_user'),
                       'oauth2server_token', type_='foreignkey')
    op.drop_index(op.f('ix_oauth2server_token_user_id'),
                  table_name='oauth2server_token')
    op.create_foreign_key('fk_oauth2server_token_user_id_accounts_user',
                          'oauth2server_token', 'accounts_user', ['user_id'],
                          ['id'])
    op.drop_constraint(
        op.f('fk_oauth2server_token_client_id_oauth2server_client'),
        'oauth2server_token', type_='foreignkey')
    op.drop_index(op.f('ix_oauth2server_token_client_id'),
                  table_name='oauth2server_token')
    op.create_foreign_key(
        'fk_oauth2server_token_client_id_oauth2server_client',
        'oauth2server_token', 'oauth2server_client', ['client_id'],
        ['client_id'])
    op.drop_constraint(op.f('fk_oauth2server_client_user_id_accounts_user'),
                       'oauth2server_client', type_='foreignkey')
    op.drop_index(op.f('ix_oauth2server_client_user_id'),
                  table_name='oauth2server_client')
    op.create_foreign_key('fk_oauth2server_client_user_id_accounts_user',
                          'oauth2server_client', 'accounts_user', ['user_id'],
                          ['id'])