[2.0.x] Fixed #28781 -- Added QuerySet.values()/values_list() support for union(), difference(), and intersection().

Thanks Tim Graham for the review.
Backport of 2d3cc94284 from master

django/db/models/sql/compiler.py

@@ -407,6 +407,11 @@ class SQLCompiler:
         parts = ()
         for compiler in compilers:
             try:
+                # If the columns list is limited, then all combined queries
+                # must have the same columns list. Set the selects defined on
+                # the query on all combined queries, if not already set.
+                if not compiler.query.values_select and self.query.values_select:
+                    compiler.query.set_values(self.query.values_select)
                 parts += (compiler.as_sql(),)
             except EmptyResultSet:
                 # Omit the empty queryset with UNION and with DIFFERENCE if the

docs/ref/models/querysets.txt

@@ -823,10 +823,17 @@ duplicate values, use the ``all=True`` argument.
 of the type of the first ``QuerySet`` even if the arguments are ``QuerySet``\s
 of other models. Passing different models works as long as the ``SELECT`` list
 is the same in all ``QuerySet``\s (at least the types, the names don't matter
-as long as the types in the same order).
+as long as the types in the same order). In such cases, you must use the column
+names from the first ``QuerySet`` in ``QuerySet`` methods applied to the
+resulting ``QuerySet``. For example::
 
-In addition, only ``LIMIT``, ``OFFSET``, ``COUNT(*)``, and ``ORDER BY`` (i.e.
+    >>> qs1 = Author.objects.values_list('name')
-slicing, :meth:`count`, and :meth:`order_by`) are allowed on the resulting
+    >>> qs2 = Entry.objects.values_list('headline')
+    >>> qs1.union(qs2).order_by('name')
+
+In addition, only ``LIMIT``, ``OFFSET``, ``COUNT(*)``, ``ORDER BY``, and
+specifying columns (i.e. slicing, :meth:`count`, :meth:`order_by`, and
+:meth:`values()`/:meth:`values_list()`) are allowed on the resulting
 ``QuerySet``. Further, databases place restrictions on what operations are
 allowed in the combined queries. For example, most databases don't allow
 ``LIMIT`` or ``OFFSET`` in the combined queries.

docs/releases/1.11.8.txt

@@ -11,3 +11,7 @@ Bugfixes
 
 * Reallowed, following a regression in Django 1.10, ``AuthenticationForm`` to
   raise the inactive user error when using ``ModelBackend`` (:ticket:`28645`).
+
+* Added support for ``QuerySet.values()`` and ``values_list()`` for
+  ``union()``, ``difference()``, and ``intersection()`` queries
+  (:ticket:`28781`).

tests/queries/test_qs_combinators.py

@@ -30,6 +30,16 @@ class QuerySetSetOperationTests(TestCase):
         qs3 = Number.objects.filter(num__gte=4, num__lte=6)
         self.assertNumbersEqual(qs1.intersection(qs2, qs3), [5], ordered=False)
 
+    @skipUnlessDBFeature('supports_select_intersection')
+    def test_intersection_with_values(self):
+        ReservedName.objects.create(name='a', order=2)
+        qs1 = ReservedName.objects.all()
+        reserved_name = qs1.intersection(qs1).values('name', 'order', 'id').get()
+        self.assertEqual(reserved_name['name'], 'a')
+        self.assertEqual(reserved_name['order'], 2)
+        reserved_name = qs1.intersection(qs1).values_list('name', 'order', 'id').get()
+        self.assertEqual(reserved_name[:2], ('a', 2))
+
     @skipUnlessDBFeature('supports_select_difference')
     def test_simple_difference(self):
         qs1 = Number.objects.filter(num__lte=5)
@@ -66,6 +76,17 @@ class QuerySetSetOperationTests(TestCase):
         self.assertEqual(len(qs2.difference(qs2)), 0)
         self.assertEqual(len(qs3.difference(qs3)), 0)
 
+    @skipUnlessDBFeature('supports_select_difference')
+    def test_difference_with_values(self):
+        ReservedName.objects.create(name='a', order=2)
+        qs1 = ReservedName.objects.all()
+        qs2 = ReservedName.objects.none()
+        reserved_name = qs1.difference(qs2).values('name', 'order', 'id').get()
+        self.assertEqual(reserved_name['name'], 'a')
+        self.assertEqual(reserved_name['order'], 2)
+        reserved_name = qs1.difference(qs2).values_list('name', 'order', 'id').get()
+        self.assertEqual(reserved_name[:2], ('a', 2))
+
     def test_union_with_empty_qs(self):
         qs1 = Number.objects.all()
         qs2 = Number.objects.none()
@@ -89,6 +110,15 @@ class QuerySetSetOperationTests(TestCase):
         qs2 = Number.objects.filter(num__gte=2, num__lte=3)
         self.assertNumbersEqual(qs1.union(qs2).order_by('-num'), [3, 2, 1, 0])
 
+    def test_union_with_values(self):
+        ReservedName.objects.create(name='a', order=2)
+        qs1 = ReservedName.objects.all()
+        reserved_name = qs1.union(qs1).values('name', 'order', 'id').get()
+        self.assertEqual(reserved_name['name'], 'a')
+        self.assertEqual(reserved_name['order'], 2)
+        reserved_name = qs1.union(qs1).values_list('name', 'order', 'id').get()
+        self.assertEqual(reserved_name[:2], ('a', 2))
+
     def test_count_union(self):
         qs1 = Number.objects.filter(num__lte=1).values('num')
         qs2 = Number.objects.filter(num__gte=2, num__lte=3).values('num')