Fixed #19816 -- Pre-evaluate querysets used in direct relation assignments.

Since assignments on M2M or reverse FK descriptors is composed of a `clear()`,
followed by an `add()`, `clear()` could potentially affect the value of the
assigned queryset before the `add()` step; pre-evaluating it solves the problem.

This patch fixes the issue for ForeignRelatedObjectsDescriptor,
ManyRelatedObjectsDescriptor, and ReverseGenericRelatedObjectsDescriptor.
It completes 6cb6e1 which addressed ReverseManyRelatedObjectsDescriptor.

4 files changed, 51 insertions, 12 deletions

diff --git a/tests/generic_relations/tests.py b/tests/generic_relations/tests.py
index ae90ace34a..b6f429cfa9 100644
--- a/tests/generic_relations/tests.py
+++ b/tests/generic_relations/tests.py
@@ -163,6 +163,21 @@ class GenericRelationsTests(TestCase):
             "<Animal: Platypus>"
         ])
 
+    def test_assign_with_queryset(self):
+        # Ensure that querysets used in reverse GFK assignments are pre-evaluated
+        # so their value isn't affected by the clearing operation in
+        # ManyRelatedObjectsDescriptor.__set__. Refs #19816.
+        bacon = Vegetable.objects.create(name="Bacon", is_yucky=False)
+        bacon.tags.create(tag="fatty")
+        bacon.tags.create(tag="salty")
+        self.assertEqual(2, bacon.tags.count())
+
+        qs = bacon.tags.filter(tag="fatty")
+        bacon.tags = qs
+
+        self.assertEqual(1, bacon.tags.count())
+        self.assertEqual(1, qs.count())
+
     def test_generic_relation_related_name_default(self):
         # Test that GenericRelation by default isn't usable from
         # the reverse side.
diff --git a/tests/m2m_regress/tests.py b/tests/m2m_regress/tests.py
index d1f85ace3e..105e41b644 100644
--- a/tests/m2m_regress/tests.py
+++ b/tests/m2m_regress/tests.py
@@ -97,15 +97,3 @@ class M2MRegressionTests(TestCase):
         # causes a TypeError in add_lazy_relation
         m1 = RegressionModelSplit(name='1')
         m1.save()
-
-    def test_m2m_filter(self):
-        worksheet = Worksheet.objects.create(id=1)
-        line_hi = Line.objects.create(name="hi")
-        line_bye = Line.objects.create(name="bye")
-
-        worksheet.lines = [line_hi, line_bye]
-        hi = worksheet.lines.filter(name="hi")
-
-        worksheet.lines = hi
-        self.assertEqual(1, worksheet.lines.count())
-        self.assertEqual(1, hi.count())
diff --git a/tests/many_to_many/tests.py b/tests/many_to_many/tests.py
index 545d50a90e..23c6cd543e 100644
--- a/tests/many_to_many/tests.py
+++ b/tests/many_to_many/tests.py
@@ -370,6 +370,30 @@ class ManyToManyTests(TestCase):
         self.assertQuerysetEqual(self.a4.publications.all(),
                                  ['<Publication: Science Weekly>'])
 
+    def test_forward_assign_with_queryset(self):
+        # Ensure that querysets used in m2m assignments are pre-evaluated
+        # so their value isn't affected by the clearing operation in
+        # ManyRelatedObjectsDescriptor.__set__. Refs #19816.
+        self.a1.publications = [self.p1, self.p2]
+
+        qs = self.a1.publications.filter(id=self.a1.id)
+        self.a1.publications = qs
+
+        self.assertEqual(1, self.a1.publications.count())
+        self.assertEqual(1, qs.count())
+
+    def test_reverse_assign_with_queryset(self):
+        # Ensure that querysets used in M2M assignments are pre-evaluated
+        # so their value isn't affected by the clearing operation in
+        # ReverseManyRelatedObjectsDescriptor.__set__. Refs #19816.
+        self.p1.article_set = [self.a1, self.a2]
+
+        qs = self.p1.article_set.filter(id=self.p1.id)
+        self.p1.article_set = qs
+
+        self.assertEqual(1, self.p1.article_set.count())
+        self.assertEqual(1, qs.count())
+
     def test_clear(self):
         # Relation sets can be cleared:
         self.p2.article_set.clear()
diff --git a/tests/many_to_one_null/tests.py b/tests/many_to_one_null/tests.py
index cd1800d77c..f3988cce7f 100644
--- a/tests/many_to_one_null/tests.py
+++ b/tests/many_to_one_null/tests.py
@@ -86,6 +86,18 @@ class ManyToOneNullTests(TestCase):
         self.assertQuerysetEqual(Article.objects.filter(reporter__isnull=True),
                                  ['<Article: First>', '<Article: Fourth>'])
 
+    def test_assign_with_queryset(self):
+        # Ensure that querysets used in reverse FK assignments are pre-evaluated
+        # so their value isn't affected by the clearing operation in
+        # ForeignRelatedObjectsDescriptor.__set__. Refs #19816.
+        self.r2.article_set = [self.a2, self.a3]
+
+        qs = self.r2.article_set.filter(id=self.a2.id)
+        self.r2.article_set = qs
+
+        self.assertEqual(1, self.r2.article_set.count())
+        self.assertEqual(1, qs.count())
+
     def test_clear_efficiency(self):
         r = Reporter.objects.create()
         for _ in range(3):