[2.0.x] Fixed #29067 -- Fixed regression in QuerySet.values_list(..., flat=True) followed by annotate().

Regression in 4dfd6b88d5.

Backport of 3187c89d6f from master

django/db/models/query.py

@@ -7,7 +7,6 @@ import operator
 import warnings
 from collections import OrderedDict, namedtuple
 from functools import lru_cache
-from itertools import chain
 
 from django.conf import settings
 from django.core import exceptions
@@ -176,7 +175,8 @@ class FlatValuesListIterable(BaseIterable):
     def __iter__(self):
         queryset = self.queryset
         compiler = queryset.query.get_compiler(queryset.db)
-        return chain.from_iterable(compiler.results_iter(chunked_fetch=self.chunked_fetch, chunk_size=self.chunk_size))
+        for row in compiler.results_iter(chunked_fetch=self.chunked_fetch, chunk_size=self.chunk_size):
+            yield row[0]
 
 
 class QuerySet:

docs/releases/2.0.2.txt

@@ -14,3 +14,6 @@ Bugfixes
 
 * Fixed incorrect foreign key nullification if a model has two foreign keys to
   the same model and a target model is deleted (:ticket:`29016`).
+
+* Fixed regression in the use of ``QuerySet.values_list(..., flat=True)``
+  followed by ``annotate()`` (:ticket:`29067`).

tests/aggregation_regress/tests.py

@@ -1475,6 +1475,11 @@ class AggregationTests(TestCase):
         vals2 = Book.objects.aggregate(result=Sum('rating') - Value(4.0))
         self.assertEqual(vals1, vals2)
 
+    def test_annotate_values_list_flat(self):
+        """Find ages that are shared by at least two authors."""
+        qs = Author.objects.values_list('age', flat=True).annotate(age_count=Count('age')).filter(age_count__gt=1)
+        self.assertSequenceEqual(qs, [29])
+
 
 class JoinPromotionTests(TestCase):
     def test_ticket_21150(self):