user = models.ForeignKey(User, unique=True)
    activation_key = models.CharField(maxlength=40)
    mobile_number = models.CharField(maxlength=75)
    birth_date = models.DateField()

    objects = RegistrationManager()

    class Admin:
        list_display = ('__str__', 'activation_key_expired')
        search_fields = ('user__username', 'user__first_name')
        list_filter = ('user__date_joined',)