[1.11.x] Fixed #28293 -- Fixed union(), intersection(), and difference() when combining with an EmptyQuerySet.

Thanks Jon Dufresne for the report and Tim Graham for the review.

Backport of 82175ead72 from master

django/db/models/query.py

@@ -838,12 +838,25 @@ class QuerySet(object):
                     "union() received an unexpected keyword argument '%s'" %
                     (unexpected_kwarg,)
                 )
+        # If the query is an EmptyQuerySet, combine all nonempty querysets.
+        if isinstance(self, EmptyQuerySet):
+            qs = [q for q in other_qs if not isinstance(q, EmptyQuerySet)]
+            return qs[0]._combinator_query('union', *qs[1:], **kwargs) if qs else self
         return self._combinator_query('union', *other_qs, **kwargs)
 
     def intersection(self, *other_qs):
+        # If any query is an EmptyQuerySet, return it.
+        if isinstance(self, EmptyQuerySet):
+            return self
+        for other in other_qs:
+            if isinstance(other, EmptyQuerySet):
+                return other
         return self._combinator_query('intersection', *other_qs)
 
     def difference(self, *other_qs):
+        # If the query is an EmptyQuerySet, return it.
+        if isinstance(self, EmptyQuerySet):
+            return self
         return self._combinator_query('difference', *other_qs)
 
     def select_for_update(self, nowait=False, skip_locked=False):

django/db/models/sql/compiler.py

@@ -379,7 +379,7 @@ class SQLCompiler(object):
         features = self.connection.features
         compilers = [
             query.get_compiler(self.using, self.connection)
-            for query in self.query.combined_queries
+            for query in self.query.combined_queries if not query.is_empty()
         ]
         if not features.supports_slicing_ordering_in_compound:
             for query, compiler in zip(self.query.combined_queries, compilers):

docs/releases/1.11.3.txt

@@ -29,3 +29,6 @@ Bugfixes
 
 * Fixed crash in admin's inlines when a model has an inherited non-editable
   primary key (:ticket:`27967`).
+
+* Fixed ``QuerySet.union()``, ``intersection()``, and ``difference()`` when
+  combining with an ``EmptyQuerySet`` (:ticket:`28293`).

tests/queries/test_qs_combinators.py

@@ -45,6 +45,31 @@ class QuerySetSetOperationTests(TestCase):
         self.assertEqual(len(list(qs1.union(qs2, all=True))), 20)
         self.assertEqual(len(list(qs1.union(qs2))), 10)
 
+    @skipUnlessDBFeature('supports_select_intersection')
+    def test_intersection_with_empty_qs(self):
+        qs1 = Number.objects.all()
+        qs2 = Number.objects.none()
+        self.assertEqual(len(qs1.intersection(qs2)), 0)
+        self.assertEqual(len(qs2.intersection(qs1)), 0)
+        self.assertEqual(len(qs2.intersection(qs2)), 0)
+
+    @skipUnlessDBFeature('supports_select_difference')
+    def test_difference_with_empty_qs(self):
+        qs1 = Number.objects.all()
+        qs2 = Number.objects.none()
+        self.assertEqual(len(qs1.difference(qs2)), 10)
+        self.assertEqual(len(qs2.difference(qs1)), 0)
+        self.assertEqual(len(qs2.difference(qs2)), 0)
+
+    def test_union_with_empty_qs(self):
+        qs1 = Number.objects.all()
+        qs2 = Number.objects.none()
+        self.assertEqual(len(qs1.union(qs2)), 10)
+        self.assertEqual(len(qs2.union(qs1)), 10)
+        self.assertEqual(len(qs2.union(qs1, qs1, qs1)), 10)
+        self.assertEqual(len(qs2.union(qs1, qs1, all=True)), 20)
+        self.assertEqual(len(qs2.union(qs2)), 0)
+
     def test_union_bad_kwarg(self):
         qs1 = Number.objects.all()
         msg = "union() received an unexpected keyword argument 'bad'"