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

Author: malcolmt

django/contrib/gis/tests/relatedapp/tests.py

@@ -184,12 +184,12 @@ def test07_values(self):
             self.assertEqual(m.point, t[1])
 
     # Test disabled until #10572 is resolved.
-    #def test08_defer_only(self):
-    #    "Testing defer() and only() on Geographic models."
-    #    qs = Location.objects.all()
-    #    def_qs = Location.objects.defer('point')
-    #    for loc, def_loc in zip(qs, def_qs):
-    #        self.assertEqual(loc.point, def_loc.point)
 
     # TODO: Related tests for KML, GML, and distance lookups.
 

django/db/models/base.py

@@ -362,9 +362,8 @@ def __reduce__(self):
                     # DeferredAttribute classes, so we only need to do this
                     # once.
                     obj = self.__class__.__dict__[field.attname]
-                    pk_val = obj.pk_value
                     model = obj.model_ref()
-        return (model_unpickle, (model, pk_val, defers), data)
 
     def _get_pk_val(self, meta=None):
         if not meta:
@@ -635,12 +634,12 @@ def get_absolute_url(opts, func, self, *args, **kwargs):
 class Empty(object):
     pass
 
-def model_unpickle(model, pk_val, attrs):
     """
     Used to unpickle Model subclasses with deferred fields.
     """
     from django.db.models.query_utils import deferred_class_factory
-    cls = deferred_class_factory(model, pk_val, attrs)
     return cls.__new__(cls)
 model_unpickle.__safe_for_unpickle__ = True
 

django/db/models/query.py

@@ -190,32 +190,31 @@ def iterator(self):
         index_start = len(extra_select)
         aggregate_start = index_start + len(self.model._meta.fields)
 
+        load_fields = only_load.get(self.model)
+        skip = None
+        if load_fields and not fill_cache:
+            # Some fields have been deferred, so we have to initialise
+            # via keyword arguments.
+            skip = set()
+            init_list = []
+            for field in fields:
+                if field.name not in load_fields:
+                    skip.add(field.attname)
+                else:
+                    init_list.append(field.attname)
+            model_cls = deferred_class_factory(self.model, skip)
+
         for row in self.query.results_iter():
             if fill_cache:
                 obj, _ = get_cached_row(self.model, row,
                             index_start, max_depth,
                             requested=requested, offset=len(aggregate_select),
                             only_load=only_load)
             else:
-                load_fields = only_load.get(self.model)
-                if load_fields:
-                    # Some fields have been deferred, so we have to initialise
-                    # via keyword arguments.
                     row_data = row[index_start:aggregate_start]
                     pk_val = row_data[pk_idx]
-                    skip = set()
-                    init_list = []
-                    for field in fields:
-                        if field.name not in load_fields:
-                            skip.add(field.attname)
-                        else:
-                            init_list.append(field.attname)
-                    if skip:
-                        model_cls = deferred_class_factory(self.model, pk_val,
-                                skip)
-                        obj = model_cls(**dict(zip(init_list, row_data)))
-                    else:
-                        obj = self.model(*row[index_start:aggregate_start])
                 else:
                     # Omit aggregates in object creation.
                     obj = self.model(*row[index_start:aggregate_start])
@@ -927,7 +926,7 @@ def get_cached_row(klass, row, index_start, max_depth=0, cur_depth=0,
                 else:
                     init_list.append(field.attname)
             if skip:
-                klass = deferred_class_factory(klass, pk_val, skip)
                 obj = klass(**dict(zip(init_list, fields)))
             else:
                 obj = klass(*fields)

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):
         self.field_name = field_name
-        self.pk_value = pk_value
         self.model_ref = weakref.ref(model)
         self.loaded = False
 
@@ -170,21 +169,18 @@ def __get__(self, instance, owner):
         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
-
-    def __set__(self, name, value):
         """
         Deferred loading attributes can be set normally (which means there will
         never be a database lookup involved.
         """
-        self.value = value
-        self.loaded = True
 
 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):
     """
     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 @@ class Meta:
     # are identical.
     name = "%s_Deferred_%s" % (model.__name__, '_'.join(sorted(list(attrs))))
 
-    overrides = dict([(attr, DeferredAttribute(attr, pk_value, model))
             for attr in attrs])
     overrides["Meta"] = Meta
     overrides["__module__"] = model.__module__
@@ -233,4 +229,3 @@ class Meta:
 # The above function is also used to unpickle model instances with deferred
 # fields.
 deferred_class_factory.__safe_for_unpickling__ = True
-

tests/regressiontests/defer_regress/models.py

@@ -6,14 +6,17 @@
 from django.db import connection, models
 
 class Item(models.Model):
-    name = models.CharField(max_length=10)
     text = models.TextField(default="xyzzy")
     value = models.IntegerField()
     other_value = models.IntegerField(default=0)
 
     def __unicode__(self):
         return self.name
 
+class RelatedItem(models.Model):
+    item = models.ForeignKey(Item)
+
 __test__ = {"regression_tests": """
 Deferred fields should really be deferred and not accidentally use the field's
 default value just because they aren't passed to __init__.
@@ -39,9 +42,31 @@ def __unicode__(self):
 u"xyzzy"
 >>> len(connection.queries) == num + 2      # Effect of text lookup.
 True
+>>> obj.text
+u"xyzzy"
+>>> len(connection.queries) == num + 2
+True
 
 >>> settings.DEBUG = False
 
+Regression test for #10695. Make sure different instances don't inadvertently
+share data in the deferred descriptor objects.
+
+>>> i = Item.objects.create(name="no I'm first", value=37)
+>>> items = Item.objects.only('value').order_by('-value')
+>>> items[0].name
+u'first'
+>>> items[1].name
+u"no I'm first"
+
+>>> _ = RelatedItem.objects.create(item=i)
+>>> r = RelatedItem.objects.defer('item').get()
+>>> r.item_id == i.id
+True
+>>> r.item == i
+True
+
+
+
 """
 }
-