Django custom model manager chaining

django_custom_model_manager_chaining.py

"""Copied from http://hunterford.me/django-custom-model-manager-chaining/
Reference: http://www.dabapps.com/blog/higher-level-query-api-django-orm/

usage: 
    Post.objects.published()
    Post.objects.by_author(user=request.user).published()
"""
from django.db import models


class PostQuerySetMixin(object):
    def by_author(self, user):
        return self.filter(user=user)

    def published(self):
        return self.filter(published__lte=datetime.now())


class PostQuerySet(PostQuerySetMixin, models.query.QuerySet):
    pass


class PostManager(PostQuerySetMixin, models.Manager):
    def get_query_set(self):
        return PostQuerySet(self.model, using=self._db)


class Post(models.Model):
    user = models.ForeignKey(User)
    published = models.DateTimeField()

    objects = PostManager()