Fixed #18177 -- Cached known related instances.

This was recently fixed for one-to-one relations; this patch adds
support for foreign keys. Thanks kaiser.yann for the report and
the initial version of the patch.

tests/modeltests/known_related_objects/fixtures/tournament.json

@@ -0,0 +1,65 @@
+[
+    {
+        "pk": 1,
+        "model": "known_related_objects.tournament",
+        "fields": {
+            "name": "Tourney 1"
+            }
+        },
+    {
+        "pk": 2,
+        "model": "known_related_objects.tournament",
+        "fields": {
+            "name": "Tourney 2"
+            }
+        },
+    {
+        "pk": 1,
+        "model": "known_related_objects.pool",
+        "fields": {
+            "tournament": 1,
+            "name": "T1 Pool 1"
+            }
+        },
+    {
+        "pk": 2,
+        "model": "known_related_objects.pool",
+        "fields": {
+            "tournament": 1,
+            "name": "T1 Pool 2"
+            }
+        },
+    {
+        "pk": 3,
+        "model": "known_related_objects.pool",
+        "fields": {
+            "tournament": 2,
+            "name": "T2 Pool 1"
+            }
+        },
+    {
+        "pk": 4,
+        "model": "known_related_objects.pool",
+        "fields": {
+            "tournament": 2,
+            "name": "T2 Pool 2"
+            }
+        },
+    {
+        "pk": 1,
+        "model": "known_related_objects.poolstyle",
+        "fields": {
+            "name": "T1 Pool 2 Style",
+            "pool": 2
+            }
+        },
+    {
+        "pk": 2,
+        "model": "known_related_objects.poolstyle",
+        "fields": {
+            "name": "T2 Pool 1 Style",
+            "pool": 3
+            }
+        }
+]
+

tests/modeltests/known_related_objects/models.py

@@ -0,0 +1,19 @@
+"""
+Existing related object instance caching.
+
+Test that queries are not redone when going back through known relations.
+"""
+
+from django.db import models
+
+class Tournament(models.Model):
+    name = models.CharField(max_length=30)
+
+class Pool(models.Model):
+    name = models.CharField(max_length=30)
+    tournament = models.ForeignKey(Tournament)
+
+class PoolStyle(models.Model):
+    name = models.CharField(max_length=30)
+    pool = models.OneToOneField(Pool)
+

tests/modeltests/known_related_objects/tests.py

@@ -0,0 +1,88 @@
+from __future__ import absolute_import
+
+from django.test import TestCase
+
+from .models import Tournament, Pool, PoolStyle
+
+class ExistingRelatedInstancesTests(TestCase):
+    fixtures = ['tournament.json']
+
+    def test_foreign_key(self):
+        with self.assertNumQueries(2):
+            tournament = Tournament.objects.get(pk=1)
+            pool = tournament.pool_set.all()[0]
+            self.assertIs(tournament, pool.tournament)
+
+    def test_foreign_key_prefetch_related(self):
+        with self.assertNumQueries(2):
+            tournament = (Tournament.objects.prefetch_related('pool_set').get(pk=1))
+            pool = tournament.pool_set.all()[0]
+            self.assertIs(tournament, pool.tournament)
+
+    def test_foreign_key_multiple_prefetch(self):
+        with self.assertNumQueries(2):
+            tournaments = list(Tournament.objects.prefetch_related('pool_set'))
+            pool1 = tournaments[0].pool_set.all()[0]
+            self.assertIs(tournaments[0], pool1.tournament)
+            pool2 = tournaments[1].pool_set.all()[0]
+            self.assertIs(tournaments[1], pool2.tournament)
+
+    def test_one_to_one(self):
+        with self.assertNumQueries(2):
+            style = PoolStyle.objects.get(pk=1)
+            pool = style.pool
+            self.assertIs(style, pool.poolstyle)
+
+    def test_one_to_one_select_related(self):
+        with self.assertNumQueries(1):
+            style = PoolStyle.objects.select_related('pool').get(pk=1)
+            pool = style.pool
+            self.assertIs(style, pool.poolstyle)
+
+    def test_one_to_one_multi_select_related(self):
+        with self.assertNumQueries(1):
+            poolstyles = list(PoolStyle.objects.select_related('pool'))
+            self.assertIs(poolstyles[0], poolstyles[0].pool.poolstyle)
+            self.assertIs(poolstyles[1], poolstyles[1].pool.poolstyle)
+
+    def test_one_to_one_prefetch_related(self):
+        with self.assertNumQueries(2):
+            style = PoolStyle.objects.prefetch_related('pool').get(pk=1)
+            pool = style.pool
+            self.assertIs(style, pool.poolstyle)
+
+    def test_one_to_one_multi_prefetch_related(self):
+        with self.assertNumQueries(2):
+            poolstyles = list(PoolStyle.objects.prefetch_related('pool'))
+            self.assertIs(poolstyles[0], poolstyles[0].pool.poolstyle)
+            self.assertIs(poolstyles[1], poolstyles[1].pool.poolstyle)
+
+    def test_reverse_one_to_one(self):
+        with self.assertNumQueries(2):
+            pool = Pool.objects.get(pk=2)
+            style = pool.poolstyle
+            self.assertIs(pool, style.pool)
+
+    def test_reverse_one_to_one_select_related(self):
+        with self.assertNumQueries(1):
+            pool = Pool.objects.select_related('poolstyle').get(pk=2)
+            style = pool.poolstyle
+            self.assertIs(pool, style.pool)
+
+    def test_reverse_one_to_one_prefetch_related(self):
+        with self.assertNumQueries(2):
+            pool = Pool.objects.prefetch_related('poolstyle').get(pk=2)
+            style = pool.poolstyle
+            self.assertIs(pool, style.pool)
+
+    def test_reverse_one_to_one_multi_select_related(self):
+        with self.assertNumQueries(1):
+            pools = list(Pool.objects.select_related('poolstyle'))
+            self.assertIs(pools[1], pools[1].poolstyle.pool)
+            self.assertIs(pools[2], pools[2].poolstyle.pool)
+
+    def test_reverse_one_to_one_multi_prefetch_related(self):
+        with self.assertNumQueries(2):
+            pools = list(Pool.objects.prefetch_related('poolstyle'))
+            self.assertIs(pools[1], pools[1].poolstyle.pool)
+            self.assertIs(pools[2], pools[2].poolstyle.pool)