from datetime import datetime

from django.db import models
from django.db.models.query import QuerySet

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

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

class PostQuerySet(QuerySet, PostMixin):
    pass

class PostManager(models.Manager, PostMixin):
    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()

Post.objects.by_author(user=request.user).published()