managers.py

from __future__ import unicode_literals

from django.db import models


class QuerySetClassManager(models.Manager):
    """
    A mixin that refers to it's queryset_class attribute to create it.
    """
    queryset_class = models.query.QuerySet

    def get_queryset(self, *args, **kwargs):
        return self.queryset_class(self.model, using=self._db, *args, **kwargs)


class QuerySetProxyManager(models.Manager):
    """
    A mixin that fallbacks to queryset attributes when they are missing.
    """
    def __getattr__(self, name):
        try:
            return getattr(self.__class__, name)
        except AttributeError:
            if name.startswith('__'):  # Don't proxy reserved members
                raise
            return getattr(self.get_queryset(), name)


def queryset_manager_factory(queryset_class, bases=None, **attrs):
    """
    A factory that creates a manager class proxying a query set subclass.
    """
    # Try to figure out a decent manager name
    name = queryset_class.__name__
    if 'QuerySet' in name:
        name = str(name.replace('QuerySet', 'Manager'))
    else:
        name = str("%sManager" % name)
    # Make sure the manager subclass both `QuerySetClassManager` and
    # QuerySetProxyManager
    if bases is None:
        bases = (QuerySetClassManager, QuerySetProxyManager)
    else:
        if not any(issubclass(base, QuerySetClassManager) for base in bases):
            bases = (QuerySetClassManager,) + bases
        if not any(issubclass(base, QuerySetProxyManager) for base in bases):
            bases = (QuerySetProxyManager,) + bases
    # Added the appropriate attributes
    attrs.update(queryset_class=queryset_class)
    return type(name, bases, attrs)