Fix #203 -- Prevent access to deferred fields in stdimage file descri…

…ptor (#204)

stdimage/models.py

@@ -240,7 +240,8 @@ def set_variations(self, instance=None, **kwargs):
 
         :param instance: FileField
         """
-        if getattr(instance, self.name):
+        deferred_field = self.name in instance.get_deferred_fields()
+        if not deferred_field and getattr(instance, self.name):
             field = getattr(instance, self.name)
             if field._committed:
                 for name, variation in list(self.variations.items()):

tests/test_models.py

@@ -167,6 +167,20 @@ def test_custom_render_variations(self, db):
         assert instance.image.thumbnail.width == 100
         assert instance.image.thumbnail.height == 100
 
+    def test_defer(self, db, django_assert_num_queries):
+        """
+        `set_variations` does not access a deferred field.
+
+        Accessing a deferred field would cause Django to do
+        a second implicit database query.
+        """
+        instance = ResizeModel.objects.create(image=self.fixtures['100.gif'])
+        with django_assert_num_queries(1):
+            deferred = ResizeModel.objects.only('pk').get(pk=instance.pk)
+        with django_assert_num_queries(1):
+            deferred.image
+        assert instance.image.thumbnail == deferred.image.thumbnail
+
 
 class TestUtils(TestStdImage):
     """Tests Utils"""