title: Using email as username field in Django 1.5 custom User model results in FieldError
body: <p>I want to use an email field as the username field for my custom user model.
  I have the following custom User model subclassing Django's AbstractUser model:</p>
  
  <pre><code>class CustomUser(AbstractUser):
      ....
      email = models.EmailField(max_length=255, unique=True)
  
      USERNAME_FIELD = 'email'
  </code></pre>
  
  <p>But when I run </p>
  
  <blockquote>
    <p>python manage.py sql myapp</p>
  </blockquote>
  
  <p>I get the following error:</p>
  
  <blockquote>
    <p>FieldError: Local field 'email' in class 'CustomUser' clashes with field of similar name from base class 'AbstractUser'</p>
  </blockquote>
  
  <p>The reason I include my own email field in the first place is to add the <code>unique=True</code> option to it. otherwise I get:</p>
  
  <blockquote>
    <p>myapp.customuser: The USERNAME_FIELD must be unique. Add unique=True to the field parameters.</p>
  </blockquote>
  
  <p>Now, in respect to this:
  <a href="https://docs.djangoproject.com/en/1.5/topics/db/models/#field-name-hiding-is-not-permitted">https://docs.djangoproject.com/en/1.5/topics/db/models/#field-name-hiding-is-not-permitted</a><br>
  How <strong>can</strong> I achieve this? (other then naming the field "user_email" or something like that instead)</p>