[1.6.x] Fixed #21150 -- Improved Query.add_fields() join promotion logic

Thanks to Eivind Fonn for the report and test case.

django/db/models/sql/query.py

@@ -1525,7 +1525,7 @@ class Query(object):
             for name in field_names:
                 field, targets, u2, joins, path = self.setup_joins(
                         name.split(LOOKUP_SEP), opts, alias, None, allow_m2m,
-                        True)
+                        allow_explicit_fk=True, outer_if_first=True)
 
                 # Trim last join if possible
                 targets, final_alias, remaining_joins = self.trim_joins(targets, joins[-2:], path)

tests/aggregation_regress/models.py

@@ -87,3 +87,14 @@ class HardbackBook(Book):
 
     def __str__(self):
         return "%s (hardback): %s" % (self.name, self.weight)
+
+# Models for ticket #21150
+class Alfa(models.Model):
+    pass
+
+class Bravo(models.Model):
+    pass
+
+class Charlie(models.Model):
+    alfa = models.ForeignKey(Alfa, null=True)
+    bravo = models.ForeignKey(Bravo, null=True)

tests/aggregation_regress/tests.py

@@ -12,8 +12,9 @@ from django.test import TestCase, Approximate, skipUnlessDBFeature
 from django.utils import six
 from django.utils.unittest import expectedFailure
 
-from .models import (Author, Book, Publisher, Clues, Entries, HardbackBook,
+from .models import (
-        ItemTag, WithManualPK)
+    Author, Book, Publisher, Clues, Entries, HardbackBook, ItemTag,
+    WithManualPK, Alfa, Bravo, Charlie)
 
 
 class AggregationTests(TestCase):
@@ -1107,3 +1108,19 @@ class AggregationTests(TestCase):
         self.assertQuerysetEqual(
             qs, ['Sams Teach Yourself Django in 24 Hours'],
             lambda b: b.name)
+
+    def test_ticket_21150(self):
+        b = Bravo.objects.create()
+        c = Charlie.objects.create(bravo=b)
+        qs = Charlie.objects.select_related('alfa').annotate(Count('bravo__charlie'))
+        self.assertQuerysetEqual(
+            qs, [c], lambda x: x)
+        self.assertIs(qs[0].alfa, None)
+        a = Alfa.objects.create()
+        c.alfa = a
+        c.save()
+        # Force re-evaluation
+        qs = qs.all()
+        self.assertQuerysetEqual(
+            qs, [c], lambda x: x)
+        self.assertEqual(qs[0].alfa, a)