django model manager example

models.py

from django.contrib.auth.models import AbstractBaseUser, PermissionsMixin, BaseUserManager
from django.db import models, transaction
from django.utils import timezone


class Manager(models.Manager):
    """ Use this class to define custom methods on models.
    """

    def get_query_set(self):
        return QuerySet(self.model)

    def __getattr__(self, name, *args):
        if name.startswith('_'):
            raise AttributeError
        return getattr(self.get_query_set(), name, *args)


""" Custom QuerySet class to provide a first() method, which behaves like
get() except returns None instead of raising an exception.
"""
class QuerySet(models.query.QuerySet):
    """ Use this class to define methods on QuerySet itself.
    """

    def first(self, *args, **kwargs):
        try:
            return self.filter(*args, **kwargs)[0]
        except IndexError:
            return None


class UserManager(BaseUserManager):

    def _create_user(self, email, first_name, last_name, password,
                     is_staff, is_superuser, **extra_fields):
        """
        Creates and saves a User with the given email and password.
        """
        now = timezone.now()
        if not email:
            raise ValueError('The given email must be set')
        email = self.normalize_email(email).lower()
        user = self.model(email=email,
                          first_name=first_name, last_name=last_name,
                          is_staff=is_staff, is_active=True,
                          is_superuser=is_superuser, last_login=now,
                          **extra_fields)
        user.set_password(password)

        user.save(using=self._db)

        return user

    def create_user(self, email, first_name=None, last_name=None, password=None,
                    **extra_fields):
        if not first_name:
            first_name = u''
        if not last_name:
            last_name = u''
        return self._create_user(email, first_name, last_name, password,
                                 False, False, **extra_fields)

    def create_superuser(self, email, first_name, last_name, password,
                         **extra_fields):
        return self._create_user(email, first_name, last_name, password,
                                 True, True, **extra_fields)


class User(AbstractBaseUser, PermissionsMixin):
    objects = UserManager()
    USERNAME_FIELD = 'email'

    email = models.EmailField(max_length=254, unique=True, db_index=True)
    first_name = models.CharField(max_length=60)
    last_name = models.CharField(max_length=60)
    is_staff = models.BooleanField(default=False,
        help_text='Designates whether the user can log into this admin '
                    'site.')
    is_active = models.BooleanField(default=True,
            help_text='Designates whether this user should be treated as '
                        'active. Unselect this instead of deleting accounts.')
    language = models.CharField(max_length=120, null=True)
    timezone = models.CharField(max_length=120, null=True)
    type = models.CharField(max_length=120, null=True)
    created_at = models.DateTimeField(auto_now_add=True)
    updated_at = models.DateTimeField(auto_now=True)


class UserProfile(models.Model):
    objects = Manager()

    user = models.ForeignKey(User, unique=True)
    birth_date = models.DateField(null=True)
    gender = models.CharField(max_length=30, null=True)
    updated_at = models.DateTimeField(auto_now=True)


# lazy create user profile on first access
User.profile = property(lambda u: UserProfile.objects.get_or_create(user=u)[0])

I tried the same using the django signals but i got an error.