Fixed #10695 -- Fixed implementation of deferred attribute retrieval.

The original implementation had a few silly bugs in it that meant that data was
not being used only on the instance of the class that it was appropriate for
(one of the traps when using class-level things). No more!

Thanks to Justin Bronn and Alex Gaynor for the patch.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@10382 bcc190cf-cafb-0310-a4f2-bffc1f526a37

1 files changed, 10 insertions, 15 deletions

diff --git a/django/db/models/query_utils.py b/django/db/models/query_utils.py
index 8baa654344..7a5ad919a1 100644
--- a/django/db/models/query_utils.py
+++ b/django/db/models/query_utils.py
@@ -158,9 +158,8 @@ class DeferredAttribute(object):
     A wrapper for a deferred-loading field. When the value is read from this
     object the first time, the query is executed.
     """
-    def __init__(self, field_name, pk_value, model):
+    def __init__(self, field_name, model):
         self.field_name = field_name
-        self.pk_value = pk_value
         self.model_ref = weakref.ref(model)
         self.loaded = False
 
@@ -170,21 +169,18 @@ class DeferredAttribute(object):
         Returns the cached value.
         """
         assert instance is not None
-        if not self.loaded:
-            obj = self.model_ref()
-            if obj is None:
-                return
-            self.value = list(obj._base_manager.filter(pk=self.pk_value).values_list(self.field_name, flat=True))[0]
-            self.loaded = True
-        return self.value
+        cls = self.model_ref()
+        data = instance.__dict__
+        if data.get(self.field_name, self) is self:
+            data[self.field_name] = cls._base_manager.filter(pk=instance.pk).values_list(self.field_name, flat=True).get()
+        return data[self.field_name]
 
-    def __set__(self, name, value):
+    def __set__(self, instance, value):
         """
         Deferred loading attributes can be set normally (which means there will
         never be a database lookup involved.
         """
-        self.value = value
-        self.loaded = True
+        instance.__dict__[self.field_name] = value
 
 def select_related_descend(field, restricted, requested):
     """
@@ -206,7 +202,7 @@ def select_related_descend(field, restricted, requested):
 # This function is needed because data descriptors must be defined on a class
 # object, not an instance, to have any effect.
 
-def deferred_class_factory(model, pk_value, attrs):
+def deferred_class_factory(model, attrs):
     """
     Returns a class object that is a copy of "model" with the specified "attrs"
     being replaced with DeferredAttribute objects. The "pk_value" ties the
@@ -223,7 +219,7 @@ def deferred_class_factory(model, pk_value, attrs):
     # are identical.
     name = "%s_Deferred_%s" % (model.__name__, '_'.join(sorted(list(attrs))))
 
-    overrides = dict([(attr, DeferredAttribute(attr, pk_value, model))
+    overrides = dict([(attr, DeferredAttribute(attr, model))
             for attr in attrs])
     overrides["Meta"] = Meta
     overrides["__module__"] = model.__module__
@@ -233,4 +229,3 @@ def deferred_class_factory(model, pk_value, attrs):
 # The above function is also used to unpickle model instances with deferred
 # fields.
 deferred_class_factory.__safe_for_unpickling__ = True
-