无法在Django中创建自定义用户模型

我有一个自定义的无密码用户模型，内置于Django 1.11中。用户模型如下所示

class User(models.Model):
    email = models.EmailField(primary_key=True)
    REQUIRED_FIELDS = []
    USERNAME_FIELD = 'email'
    is_anonymous = False
    is_authenticated = True

这是一个自定义用户模型，并依赖于自定义auth后端，如下所示

class PasswordlessAuthenticationBackend():
    def authenticate(self, uid):
        try:
            token = Token.objects.get(uid=uid)
            return User.objects.get(email=token.email)
        except User.DoesNotExist:
            return User.objects.create(email=token.email)
        except Token.DoesNotExist:
            return None

    def get_user(self, email):
        try:
            return User.objects.get(email=email)
        except User.DoesNotExist:
            return None

身份验证已注册并且可以正常工作。令牌就是这个

class Token(models.Model):
    email = models.EmailField()
    uid = models.CharField(default=uuid.uuid4, max_length=40)

问题是，当我尝试调用auth.loginmy时TestCase，它总是抛出此错误：

ValueError: The following fields do not exist in this model or are m2m fields: last_login

什么是m2m字段？如何以及在哪里指定last_login？

编辑：

测试失败看起来像这样：

from django.test import TestCase
from django.contrib.auth import get_user_model
from django.contrib import auth
from accounts.models import Token

User = get_user_model()
email = '[email protected]'

class UserModelTestcase(TestCase):
    def test_user_is_valid_with_email_only(self):
        user = User(email=email)
        user.full_clean()
    
    def test_email_is_primary_key(self):
        user = User(email=email)
        self.assertEqual(user.pk,email)

    def test_links_user_with_auto_generated_uid(self):
        token1 = Token.objects.create(email=email)
        token2 = Token.objects.create(email=email)
        self.assertNotEqual(token1.uid, token2.uid)
    
    def test_no_problem_with_auth_login(self):
        user = User.objects.create(email=email)
        user.backend = ''
        request = self.client.request().wsgi_request
        auth.login(request, user) #should not raise, fails!