我正在尝试使用django模型在数据库上创建约束，我在这里做错了什么？
我在python 3.7.3上使用django 2.2.2
这是我的模特班：
class Client(models.Model):
class Meta:
db_table = 'Client'
constraints = [
models.CheckConstraint(check=models.Q(cnpj__regex='\\d{14}'), name='valid_cnpj'),
]
id = HashidAutoField(primary_key=True)
trade_name = models.CharField(max_length=200)
cnpj = models.CharField(max_length=14)
business_name = models.CharField(max_length=200, null=True)
错误消息：AttributeError：'str'对象没有属性'connector'
完整追溯：
Running migrations:
Applying global_models.0008_auto_20190809_1747...Traceback (most recent call last):
File "manage.py", line 21, in <module>
main()
File "manage.py", line 17, in main
execute_from_command_line(sys.argv)
File "C:\Users\Celere.LAPTOP-FLPOSE6C\PycharmProjects\api_django\venv\lib\site-packages\django\core\management\__init__.py", line 381, in execute_from_command_line
utility.execute()
File "C:\Users\Celere.LAPTOP-FLPOSE6C\PycharmProjects\api_django\venv\lib\site-packages\django\core\management\__init__.py", line 375, in execute
self.fetch_command(subcommand).run_from_argv(self.argv)
File "C:\Users\Celere.LAPTOP-FLPOSE6C\PycharmProjects\api_django\venv\lib\site-packages\django\core\management\base.py", line 323, in run_from_argv
self.execute(*args, **cmd_options)
File "C:\Users\Celere.LAPTOP-FLPOSE6C\PycharmProjects\api_django\venv\lib\site-packages\django\core\management\base.py", line 364, in execute
output = self.handle(*args, **options)
File "C:\Users\Celere.LAPTOP-FLPOSE6C\PycharmProjects\api_django\venv\lib\site-packages\django\core\management\base.py", line 83, in wrapped
res = handle_func(*args, **kwargs)
File "C:\Users\Celere.LAPTOP-FLPOSE6C\PycharmProjects\api_django\venv\lib\site-packages\django\core\management\commands\migrate.py", line 234, in handle
fake_initial=fake_initial,
File "C:\Users\Celere.LAPTOP-FLPOSE6C\PycharmProjects\api_django\venv\lib\site-packages\django\db\migrations\executor.py", line 117, in migrate
state = self._migrate_all_forwards(state, plan, full_plan, fake=fake, fake_initial=fake_initial)
File "C:\Users\Celere.LAPTOP-FLPOSE6C\PycharmProjects\api_django\venv\lib\site-packages\django\db\migrations\executor.py", line 147, in _migrate_all_forwards
state = self.apply_migration(state, migration, fake=fake, fake_initial=fake_initial)
File "C:\Users\Celere.LAPTOP-FLPOSE6C\PycharmProjects\api_django\venv\lib\site-packages\django\db\migrations\executor.py", line 245, in apply_migration
state = migration.apply(state, schema_editor)
File "C:\Users\Celere.LAPTOP-FLPOSE6C\PycharmProjects\api_django\venv\lib\site-packages\django\db\migrations\migration.py", line 124, in apply
operation.database_forwards(self.app_label, schema_editor, old_state, project_state)
File "C:\Users\Celere.LAPTOP-FLPOSE6C\PycharmProjects\api_django\venv\lib\site-packages\django\db\migrations\operations\models.py", line 827, in database_forwards
schema_editor.add_constraint(model, self.constraint)
File "C:\Users\Celere.LAPTOP-FLPOSE6C\PycharmProjects\api_django\venv\lib\site-packages\django\db\backends\base\schema.py", line 343, in add_constraint
sql = constraint.create_sql(model, self)
File "C:\Users\Celere.LAPTOP-FLPOSE6C\PycharmProjects\api_django\venv\lib\site-packages\django\db\models\constraints.py", line 47, in create_sql
check = self._get_check_sql(model, schema_editor)
File "C:\Users\Celere.LAPTOP-FLPOSE6C\PycharmProjects\api_django\venv\lib\site-packages\django\db\models\constraints.py", line 37, in _get_check_sql
where = query.build_where(self.check)
File "C:\Users\Celere.LAPTOP-FLPOSE6C\PycharmProjects\api_django\venv\lib\site-packages\django\db\models\sql\query.py", line 1296, in build_where
return self._add_q(q_object, used_aliases=set(), allow_joins=False, simple_col=True)[0]
File "C:\Users\Celere.LAPTOP-FLPOSE6C\PycharmProjects\api_django\venv\lib\site-packages\django\db\models\sql\query.py", line 1302, in _add_q
connector = q_object.connector