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

Browse source 
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
This commit is contained in:
Malcolm Tredinnick 2009-04-04 03:21:31 +00:00
parent 19d1450929
commit dded5f52cc
5 changed files with 63 additions and 45 deletions
Download patch file Download diff file Expand all files Collapse all files

25
django/db/models/query_utils.py
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@ -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