django_custom_manager.py

class CustomQuerySetManager(Manager):
    """
    A re-usable Manager to access a custom QuerySet
    """
    def __getattr__(self, attr):
        # don't delegate internal methods to queryset
        # NOTE: without this, Manager._copy_to_model will end up calling
        # __getstate__ on the *queryset* which causes the qs (as `all()`)
        #  to evaluate itself as if it was being pickled (`len(self)`)
        if attr.startswith('__'):
            raise
        return getattr(self.get_query_set(), attr)

    def get_query_set(self):
        return getattr(self.model, 'QuerySet', QuerySet)(self.model, using=self._db)


class CustomModel(models.Model):

    objects = CustomQuerySetManager()
    QuerySet = CustomQuerySet

    value1 = models.IntegerField()
    value2 = models.IntegerField()


from django.test import TestCase

class CustomQuerySetManagerTestCase(TestCase):

    def test_get_queryset_no_queries(self):
        with self.assertNumQueries(0):
            CustomModel.objects.get_query_set()

    def test_filter_unevaluated_no_queries(self):
        with self.assertNumQueries(0):
            CustomModel.objects.filter(value1=2)

    def test_filter_evaluated_1_query(self):
        with self.assertNumQueries(1):
            qs = CustomModel.objects.filter(value1=2)
            list(qs)

    def test_only_unevaluated_no_queries(self):
        with self.assertNumQueries(0):
            CustomModel.objects.only('value1')

    def test_only_evaluated_1_query(self):
        with self.assertNumQueries(1):
            qs = CustomModel.objects.only('value1')
            list(qs)

"""
in the original version of CustomQuerySetManager from http://stackoverflow.com/a/2163921/202168
`test_only_evaluated_1_query` fails because 3 queries are executed instead of 1

If we inspect the two extra queries we see they look like you did
`list(CustomModel.objects.all())` ...twice!

If you have millions of rows in your db this is very bad, you try to do:
`CustomModel.objects.only('value1').filter(value2=99)`
and you end up triggering two unbounded whole-table fetches as well as your actual query!!

As mentioned in the comment above, the reason is because of some Django model internals.
When you add an `only()` clause to your queryset, Django derives a new model class from your
original model (like `CustomModel_deferrer_value1`), as a proxy model. This leads to
`Manager._copy_to_model()` method being called to copy your custom manager to the new proxy
class... this uses `copy.copy()` which internally calls the `__getstate__` method of object
in question, as when pickling.

Unfortunately the original version of CustomQuerySetManager was naïvely delegating
the `__getstate__` call through to the *queryset* instance... and, just like when you pickle
a queryset, this causes it to be evaluated.
"""