title: django can't create superuser
body: <p>I want code a blog with Django. Default User model is not suitable for my blog, I want to code my own User.</p>
  
  <pre><code>from django.contrib.auth.models import AbstractBaseUser, BaseUserManager
  from django.db import models
  
  class MyUserManager(BaseUserManager):
      def create_superuser(self, email, username, password, created_at):
          user = self.create_user(email=email,
              username=username,
              password = password,
              created_at = created_at
              )
          user.is_admin = True
          user.save(using=self._db)
          return user
  
  class MyUser(AbstractBaseUser):
      email = models.EmailField(
          verbose_name = 'email address',
          max_length = 255,
          unique = True,
          )
  
      username = models.CharField(
          max_length = 100,
          unique = True,
          db_index = True,
          )
  
      created_at = models.DateField()
  
      is_active = models.BooleanField(default = True)
      is_admin = models.BooleanField(default = False)
  
      objects = MyUserManager()
  
      USERNAME_FIELD = 'email'
      REQUIRED_FIELDS = ['username']
  
      def get_full_name(self):
          return self.email
  
      def get_short_name(self):
          return self.username
  
      def __unicode__(self):
          return self.email
  
      def has_perm(self, perm, obj=None):
          return True
  
      def has_module_perms(self, app_label):
          return True
  
      @property
      def is_staff(self):
          return self.is_admin
  </code></pre>
  
  <p>It has a mistake.<img src="https://i.stack.imgur.com/sMMi5.jpg" alt="enter image description here"></p>
  
  <p>I follow the tutorial instructions. I don't know why this problem occurs.</p>
  
  <p><em>I post MyUser model, hope can help answer this question.</em></p>
  
  <p>What can I do?</p>