from django.contrib.auth.models import AbstractUser
from django.db.models import fields


class User(AbstractUser):
    mozillians_url = fields.URLField()
    mozillians_username = fields.CharField(max_length=30, blank=True)

from django.contrib.auth.models import AbstractUser
from django.db.models import fields


class User(AbstractUser):
    mozillians_url = fields.URLField()
    mozillians_username = fields.CharField(max_length=30, blank=True)

    def __unicode__(self):
        username = self.mozillians_username or self.username
        return unicode(username)

Use mozillians_username for unicode representation.