[1.10.x] Fixed #26900 -- Fixed crash accessing deferred FileFields.

Backport of 7c33aa8a87 from master

django/db/models/fields/files.py

@@ -167,7 +167,11 @@ class FileDescriptor(object):
 
         # The instance dict contains whatever was originally assigned
         # in __set__.
-        file = instance.__dict__[self.field.name]
+        if self.field.name in instance.__dict__:
+            file = instance.__dict__[self.field.name]
+        else:
+            instance.refresh_from_db(fields=[self.field.name])
+            file = getattr(instance, self.field.name)
 
         # If this value is a string (instance.file = "path/to/file") or None
         # then we simply wrap it with the appropriate attribute class according
@@ -437,9 +441,10 @@ class ImageField(FileField):
         Dimensions can be forced to update with force=True, which is how
         ImageFileDescriptor.__set__ calls this method.
         """
-        # Nothing to update if the field doesn't have dimension fields.
+        # Nothing to update if the field doesn't have dimension fields or if
+        # the field is deferred.
         has_dimension_fields = self.width_field or self.height_field
-        if not has_dimension_fields:
+        if not has_dimension_fields or self.attname not in instance.__dict__:
             return
 
         # getattr will call the ImageFileDescriptor's __get__ method, which

tests/model_fields/test_filefield.py

@@ -50,3 +50,7 @@ class FileFieldTests(TestCase):
         d = Document.objects.create(myfile='something.txt')
         d.refresh_from_db()
         self.assertIs(d.myfile.instance, d)
+
+    def test_defer(self):
+        Document.objects.create(myfile='something.txt')
+        self.assertEqual(Document.objects.defer('myfile')[0].myfile, 'something.txt')

tests/model_fields/test_imagefield.py

@@ -188,6 +188,13 @@ class ImageFieldTests(ImageFieldTestMixin, TestCase):
         loaded_p = pickle.loads(dump)
         self.assertEqual(p.mugshot, loaded_p.mugshot)
 
+    def test_defer(self):
+        self.PersonModel.objects.create(name='Joe', mugshot=self.file1)
+        with self.assertNumQueries(1):
+            qs = list(self.PersonModel.objects.defer('mugshot'))
+        with self.assertNumQueries(0):
+            self.assertEqual(qs[0].name, 'Joe')
+
 
 @skipIf(Image is None, "Pillow is required to test ImageField")
 class ImageFieldTwoDimensionsTests(ImageFieldTestMixin, TestCase):