Fixed #19087 -- Ensured query's base table is never LOUTER joined

This fixes a regression created by join promotion logic refactoring:
01b9c3d519

Thanks to Ivan Virabyan for the report.

django/db/models/sql/query.py

@@ -702,6 +702,11 @@ class Query(object):
         aliases = list(aliases)
         while aliases:
             alias = aliases.pop(0)
+            if self.alias_map[alias].rhs_join_col is None:
+                # This is the base table (first FROM entry) - this table
+                # isn't really joined at all in the query, so we should not
+                # alter its join type.
+                continue
             parent_alias = self.alias_map[alias].lhs_alias
             parent_louter = (parent_alias
                 and self.alias_map[parent_alias].join_type == self.LOUTER)
@@ -1188,6 +1193,9 @@ class Query(object):
                     for alias in join_list:
                         if self.alias_map[alias].join_type == self.LOUTER:
                             j_col = self.alias_map[alias].rhs_join_col
+                            # The join promotion logic should never produce
+                            # a LOUTER join for the base join - assert that.
+                            assert j_col is not None
                             entry = self.where_class()
                             entry.add(
                                 (Constraint(alias, j_col, None), 'isnull', True),

tests/regressiontests/aggregation_regress/tests.py

@@ -878,3 +878,14 @@ class AggregationTests(TestCase):
             connection.ops.convert_values(testData, testField),
             testData
         )
+
+    def test_annotate_joins(self):
+        """
+        Test that the base table's join isn't promoted to LOUTER. This could
+        cause the query generation to fail if there is an exclude() for fk-field
+        in the query, too. Refs #19087.
+        """
+        qs = Book.objects.annotate(n=Count('pk'))
+        self.assertIs(qs.query.alias_map['aggregation_regress_book'].join_type, None)
+        # Check that the query executes without problems.
+        self.assertEqual(len(qs.exclude(publisher=-1)), 6)