Fixed #17439 -- Prevented spurious queries for missing objects after prefetch_related has run.

import itertools

import sys

from django.core import exceptions

from django.db import connections, router, transaction, IntegrityError

from django.db.models.fields import AutoField

from django.db.models.query_utils import (Q, select_related_descend,

                # (e.g. via select_related), or hopefully some other property

                # that doesn't support prefetching but needs to be traversed.

                # We replace the current list of parent objects with that list.

                obj_list = [getattr(obj, attr) for obj in obj_list]

                # Filter out 'None' so that we can continue with nullable

                # relations.

                obj_list = [obj for obj in obj_list if obj is not None]

                # We replace the current list of parent objects with the list

                # of related objects, filtering out empty or missing values so

                # that we can continue with nullable or reverse relations.

                new_obj_list = []

                for obj in obj_list:

                    try:

                        new_obj = getattr(obj, attr)

                    except exceptions.ObjectDoesNotExist:

                        continue

                    if new_obj is None:

                        continue

                    new_obj_list.append(new_obj)

                obj_list = new_obj_list

def get_prefetcher(instance, attr):

        vals = rel_obj_cache.get(instance_attr_val, [])

        if single:

            # Need to assign to single cache on instance

            if vals:

                setattr(obj, cache_name, vals[0])

            setattr(obj, cache_name, vals[0] if vals else None)

        else:

            # Multi, attribute represents a manager with an .all() method that

            # returns a QuerySet

        self.assertQuerysetEqual(self.book2.authors.all(), [u"<Author: Charlotte>"])

    def test_onetoone_reverse_no_match(self):

        # Regression for #17439

        with self.assertNumQueries(2):

            book = Book.objects.prefetch_related('bookwithyear').all()[0]

        with self.assertNumQueries(0):

            with self.assertRaises(BookWithYear.DoesNotExist):

                book.bookwithyear

    def test_survives_clone(self):

        with self.assertNumQueries(2):

            lists = [list(b.first_time_authors.all())