[1.11.x] Fixed #27800 -- Fixed QuerySet.annotate(Length(...)).distinct() crash.

Backport of ac5f886c56 from master

AUTHORS

@@ -456,6 +456,7 @@ answer newbie questions, and generally made Django that much better:
     Leo Shklovskii
     Leo Soto <leo.soto@gmail.com>
     lerouxb@gmail.com
+    Lex Berezhny <lex@damoti.com>
     Liang Feng <hutuworm@gmail.com>
     limodou
     Loek van Gent <loek@barakken.nl>

django/db/models/sql/query.py

@@ -1345,7 +1345,12 @@ class Query(object):
                         "querying. If it is a GenericForeignKey, consider "
                         "adding a GenericRelation." % name
                     )
-                model = field.model._meta.concrete_model
+                try:
+                    model = field.model._meta.concrete_model
+                except AttributeError:
+                    # QuerySet.annotate() may introduce fields that aren't
+                    # attached to a model.
+                    model = None
             else:
                 # We didn't find the current field, so move position back
                 # one step.

tests/annotations/tests.py

@@ -8,7 +8,7 @@ from django.db.models import (
     BooleanField, CharField, Count, DateTimeField, ExpressionWrapper, F, Func,
     IntegerField, NullBooleanField, Q, Sum, Value,
 )
-from django.db.models.functions import Lower
+from django.db.models.functions import Length, Lower
 from django.test import TestCase, skipUnlessDBFeature
 from django.utils import six
 
@@ -208,6 +208,11 @@ class NonAggregateAnnotationTestCase(TestCase):
         ).distinct('test_alias')
         self.assertEqual(len(people2), 1)
 
+        lengths = Employee.objects.annotate(
+            name_len=Length('first_name'),
+        ).distinct('name_len').values_list('name_len', flat=True)
+        self.assertSequenceEqual(lengths, [3, 7, 8])
+
     def test_filter_annotation(self):
         books = Book.objects.annotate(
             is_book=Value(1, output_field=IntegerField())