Chaining Custome QuerySet method for Django Models

model_utils.py

from django.db import models

# http://www.djangosnippets.org/snippets/562/#c673


class QuerySetManager(models.Manager):

    # http://docs.djangoproject.com/en/dev/topics/db/managers/#using-managers-for-related-object-access
    # Not working cause of:
    # http://code.djangoproject.com/ticket/9643
    use_for_related_fields = True

    def __init__(self, qs_class=models.query.QuerySet):
        self.queryset_class = qs_class
        super(QuerySetManager, self).__init__()

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

    def __getattr__(self, attr, *args):
        try:
            return getattr(self.__class__, attr, *args)
        except AttributeError:
            return getattr(self.get_query_set(), attr, *args)


class QuerySet(models.query.QuerySet):
    """Base QuerySet class for adding custom methods that are made
    available on both the manager and subsequent cloned QuerySets"""

    @classmethod
    def as_manager(cls, ManagerClass=QuerySetManager):
        return ManagerClass(cls)


"""
Example

class MerchantQuerySet(QuerySet):

    def live(self):
        return self.filter(live=True)

    def locked(self):
        return self.filter(locked=True)

    def unlocked(self):
        return self.filter(locked=False)


class Merchant(models.Model):
    """

    """

    created_on = models.DateTimeField(auto_now_add=True)
    modified = models.DateTimeField(auto_now=True)
    name = models.CharField(max_length=200)
    live = models.BooleanField(default=False)
    locked = models.BooleanField(default=True)

    objects = MerchantQuerySet.as_manager()


We can now do

Merchant.objects.live().locked()
or 
Merchant.objects.live()
or
Merchant.objects.live().unlocked()

etc ...



"""