From d43f8478478c333ed8b7e499abef17a86363d61c Mon Sep 17 00:00:00 2001
From: Lex Berezhny <lex@damoti.com>
Date: Tue, 31 Jan 2017 18:45:55 -0500
Subject: [PATCH] [1.11.x] Fixed #27800 -- Fixed
 QuerySet.annotate(Length(...)).distinct() crash.

Backport of ac5f886c5610a6bca26dab10170b445d1e9df450 from master
---
 AUTHORS                       | 1 +
 django/db/models/sql/query.py | 7 ++++++-
 tests/annotations/tests.py    | 7 ++++++-
 3 files changed, 13 insertions(+), 2 deletions(-)

diff --git a/AUTHORS b/AUTHORS
index dbc414f5a3..bf7c21be45 100644
--- a/AUTHORS
+++ b/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>
diff --git a/django/db/models/sql/query.py b/django/db/models/sql/query.py
index 5f65786373..695e52cda7 100644
--- a/django/db/models/sql/query.py
+++ b/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.
diff --git a/tests/annotations/tests.py b/tests/annotations/tests.py
index b90286b9da..36e675402c 100644
--- a/tests/annotations/tests.py
+++ b/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())