Extra SQL en Django (completo)

models.py

from django.db import models
from django.utils improt timezone

class Person(models.Model):
    first_name = models.CharField(max_length=42)
    last_name = models.CharField(max_length=42)
    birth_date = models.DateField()
    email = models.EmailField()

    @property
    def age(self):
        now = timezone.now()
        years = now.year - self.birth_date.year
        if now.month < self.birth_date.month:
            age -= 1
        elif now.month == self.birth_date.month and now.day < birth_date.day:
            age -= 1
        return age

    @classmethod
    def get_values(self):
        age_sql = "(EXTRACT(year FROM age(birth_date)) :: int)"
        return Person.objects.extra(select={'age': age_sql})\
                             .values('first_name', 'birth_date', 'age')

    @classmethod
    def sorted_by_email_provider(self):
        email_provider_sql = "(COALESCE(substring(email from '@(.*)$'), ''))"
        Person.objects.extra(select={'email_provider': email_provider_sql}).order_by('email_provider')