[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 82175ead723f8fa3f9271fbd4b24275097029aab from master

2 files changed, 14 insertions, 1 deletions

diff --git a/django/db/models/query.py b/django/db/models/query.py
index c9ff437232..8a97e45b88 100644
--- a/django/db/models/query.py
+++ b/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):
diff --git a/django/db/models/sql/compiler.py b/django/db/models/sql/compiler.py
index 9acb56aa84..d1373fcf95 100644
--- a/django/db/models/sql/compiler.py
+++ b/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):