Django 用户模型抛出重复的键值违反了用户创建时的唯一约束“user_user_username_key”

如何解决Django 用户模型抛出重复的键值违反了用户创建时的唯一约束“user_user_username_key”

我正在测试我的用户模型以查看它的信号是否正常工作，但是我收到一个错误：

psycopg2.errors.UniqueViolation: duplicate key value violates unique constraint "user_user_username_key"
DETAIL:  Key (username)=() already exists.

目前我的测试是：

class UserTestCase(TestCase):
    
    obj1 = None
    obj2 = None

    def setUp(self):
        self.obj1 = User.objects.create(first_name="james",last_name="adams",email="[e-mail]")
        self.obj2 = User.objects.create(first_name="amy",last_name="",email="[e-mail]")

我的信号文件是：

@receiver(pre_save,sender=User,dispatch_uid="set_username_of_user")
def set_username(sender,instance,**kwargs):
    """
    Every time a user is saved,ensures that user has a certain username
    This method avoids users having username field set to null
    """
    if not instance.username:
        email = instance.email
        email_without_domain = email[:email.find("@")].replace("-","_").lower().strip()[:20].replace(' ','')
        username = create_username(email_without_domain)
        print(username)
        if User.objects.filter(username=username).exists():
            while User.objects.filter(username=username):
                username = create_username(email_without_domain)
            instance.username = username
            instance.save()

我可以确认正在调用信号，但是它仍然会抛出该错误。什么可能导致此问题？