code: def upgrade():
      
      op.drop_constraint(u'fk_access_actionsusers_user_id_accounts_user',
                         'access_actionsusers', type_='foreignkey')
      op.drop_index(op.f('ix_access_actionsusers_user_id'),
                    table_name='access_actionsusers')
  
      op.alter_column('access_actionsusers', 'user_id', nullable=False,
                      existing_type=sa.Integer())
  
      op.create_index(op.f('ix_access_actionsusers_user_id'),
                      'access_actionsusers', ['user_id'], unique=False)
      op.create_foreign_key(op.f('fk_access_actionsusers_user_id_accounts_user'),
                            'access_actionsusers', 'accounts_user', ['user_id'],
                            ['id'], ondelete='CASCADE')
docs: Upgrade database.