Fixed #28378 -- Fixed union() and difference() when combining with a queryset raising EmptyResultSet.

Thanks Jon Dufresne for the report. Thanks Tim Graham and Simon Charette
for the reviews.

django/db/models/sql/compiler.py

@@ -399,7 +399,18 @@ class SQLCompiler:
                     raise DatabaseError('LIMIT/OFFSET not allowed in subqueries of compound statements.')
                 if compiler.get_order_by():
                     raise DatabaseError('ORDER BY not allowed in subqueries of compound statements.')
-        parts = (compiler.as_sql() for compiler in compilers)
+        parts = ()
+        for compiler in compilers:
+            try:
+                parts += (compiler.as_sql(),)
+            except EmptyResultSet:
+                # Omit the empty queryset with UNION and with DIFFERENCE if the
+                # first queryset is nonempty.
+                if combinator == 'union' or (combinator == 'difference' and parts):
+                    continue
+                raise
+        if not parts:
+            return [], []
         combinator_sql = self.connection.ops.set_operators[combinator]
         if all and combinator == 'union':
             combinator_sql += ' ALL'
@@ -422,16 +433,7 @@ class SQLCompiler:
         refcounts_before = self.query.alias_refcount.copy()
         try:
             extra_select, order_by, group_by = self.pre_sql_setup()
-            distinct_fields = self.get_distinct()
-
-            # This must come after 'select', 'ordering', and 'distinct' -- see
-            # docstring of get_from_clause() for details.
-            from_, f_params = self.get_from_clause()
-
             for_update_part = None
-            where, w_params = self.compile(self.where) if self.where is not None else ("", [])
-            having, h_params = self.compile(self.having) if self.having is not None else ("", [])
-
             combinator = self.query.combinator
             features = self.connection.features
             if combinator:
@@ -439,6 +441,12 @@ class SQLCompiler:
                     raise NotSupportedError('{} is not supported on this database backend.'.format(combinator))
                 result, params = self.get_combinator_sql(combinator, self.query.combinator_all)
             else:
+                distinct_fields = self.get_distinct()
+                # This must come after 'select', 'ordering', and 'distinct'
+                # (see docstring of get_from_clause() for details).
+                from_, f_params = self.get_from_clause()
+                where, w_params = self.compile(self.where) if self.where is not None else ("", [])
+                having, h_params = self.compile(self.having) if self.having is not None else ("", [])
                 result = ['SELECT']
                 params = []
 

docs/releases/1.11.4.txt

@@ -12,3 +12,6 @@ Bugfixes
 * Fixed a regression in 1.11.3 on Python 2 where non-ASCII ``format`` values
   for date/time widgets results in an empty ``value`` in the widget's HTML
   (:ticket:`28355`).
+
+* Fixed ``QuerySet.union()`` and ``difference()`` when combining with
+  a queryset raising ``EmptyResultSet`` (:ticket:`28378`).

tests/queries/test_qs_combinators.py

@@ -58,18 +58,26 @@ class QuerySetSetOperationTests(TestCase):
     def test_difference_with_empty_qs(self):
         qs1 = Number.objects.all()
         qs2 = Number.objects.none()
+        qs3 = Number.objects.filter(pk__in=[])
         self.assertEqual(len(qs1.difference(qs2)), 10)
+        self.assertEqual(len(qs1.difference(qs3)), 10)
         self.assertEqual(len(qs2.difference(qs1)), 0)
+        self.assertEqual(len(qs3.difference(qs1)), 0)
         self.assertEqual(len(qs2.difference(qs2)), 0)
+        self.assertEqual(len(qs3.difference(qs3)), 0)
 
     def test_union_with_empty_qs(self):
         qs1 = Number.objects.all()
         qs2 = Number.objects.none()
+        qs3 = Number.objects.filter(pk__in=[])
         self.assertEqual(len(qs1.union(qs2)), 10)
         self.assertEqual(len(qs2.union(qs1)), 10)
+        self.assertEqual(len(qs1.union(qs3)), 10)
+        self.assertEqual(len(qs3.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)
+        self.assertEqual(len(qs3.union(qs3)), 0)
 
     def test_limits(self):
         qs1 = Number.objects.all()