[1.5.x] Fixed #16436 -- defer + annotate + select_related crash

Correctly calculate the ``aggregate_start`` offset from loaded fields,
if any are deferred, instead of ``self.query.select`` which includes all
fields on the model.

Backpatch of 69f7db153d from master.

django/db/models/sql/compiler.py

@@ -775,7 +775,8 @@ class SQLCompiler(object):
         for rows in self.execute_sql(MULTI):
             for row in rows:
                 if has_aggregate_select:
-                    aggregate_start = len(self.query.extra_select) + len(self.query.select)
+                    loaded_fields = self.query.get_loaded_field_names().get(self.query.model, set()) or self.query.select
+                    aggregate_start = len(self.query.extra_select) + len(loaded_fields)
                     aggregate_end = aggregate_start + len(self.query.aggregate_select)
                 if resolve_columns:
                     if fields is None:

tests/modeltests/defer/models.py

@@ -28,3 +28,22 @@ class BigChild(Primary):
 class ChildProxy(Child):
     class Meta:
         proxy=True
+
+class Profile(models.Model):
+    profile1 = models.TextField(default='profile1')
+
+class Location(models.Model):
+    location1 = models.TextField(default='location1')
+
+class Item(models.Model):
+    pass
+
+class Request(models.Model):
+    profile = models.ForeignKey(Profile, null=True, blank=True)
+    location = models.ForeignKey(Location)
+    items = models.ManyToManyField(Item)
+
+    request1 = models.TextField(default='request1')
+    request2 = models.TextField(default='request2')
+    request3 = models.TextField(default='request3')
+    request4 = models.TextField(default='request4')

tests/modeltests/defer/tests.py

@@ -1,9 +1,10 @@
 from __future__ import absolute_import
 
+from django.db.models import Count
 from django.db.models.query_utils import DeferredAttribute, InvalidQuery
 from django.test import TestCase
 
-from .models import Secondary, Primary, Child, BigChild, ChildProxy
+from .models import Secondary, Primary, Child, BigChild, ChildProxy, Location, Request
 
 
 class DeferTests(TestCase):
@@ -183,3 +184,17 @@ class DeferTests(TestCase):
         with self.assertNumQueries(0):
             bc_deferred.id
         self.assertEqual(bc_deferred.pk, bc_deferred.id)
+
+class DeferAnnotateSelectRelatedTest(TestCase):
+    def test_defer_annotate_select_related(self):
+        location = Location.objects.create()
+        Request.objects.create(location=location)
+        self.assertIsInstance(list(Request.objects
+            .annotate(Count('items')).select_related('profile', 'location')
+            .only('profile', 'location')), list)
+        self.assertIsInstance(list(Request.objects
+            .annotate(Count('items')).select_related('profile', 'location')
+            .only('profile__profile1', 'location__location1')), list)
+        self.assertIsInstance(list(Request.objects
+            .annotate(Count('items')).select_related('profile', 'location')
+            .defer('request1', 'request2', 'request3', 'request4')), list)