Django QuerySet .defer（）问题 - bug或功能？ [英] Django QuerySet .defer() problem - bug or feature?

问题描述

一个例子比一千个词更好：

 在[3]中：User.objects.filter（id = 19 ）[0] == User.objects.filter（id = 19）[0] 
 Out [3]：True 
 
在[4]中：User.objects.filter（id = 19）[0] == User.objects.filter（id = 19）.defer（'email'）[0] 
 Out [4]：False 
  pre> 
 
 
它是否按目的工作？
 
 
 
 Subquestion：有什么简单的方法来获得正常的模型实例从延期的一个？
 
 
 
编辑：
 
 
 
看起来像contenttypes框架是适当修补的：
  http://code.djangoproject.com/changeset/10523  
 
 
 
所以我会说，模型._____ eq _____（）运算符不应该像此：
  def __eq __（self ，其他）：
 return isinstance（other，self .__ class__）和self._get_pk_val（）== other._get_pk_val（）
  
，但更像这样：
  def __eq __（self，other）：
返回ContentType.objects.get_for_model（self）是ContentType.objects.get_for_model（other）和self._get_pk_val（）== other._get_pk_val（）
  
这当然是引起两个DB命中，但幸运的是get_for_model似乎实现缓存。
解决方案
延迟查询返回一个不同的类，由 deferred_class_factory 提供：
 ＃在db / models / query_utils.py 
 
 def deferred_class_factory（model，attrs）：

返回一个类对象，模型与指定的attrs
替换为DeferredAttribute对象。 pk_value将
延迟属性与模型的特定实例相关联。 

  
它基本上是一个代理，你可以从方法解析中看到订单：
 >>> x = User.objects.filter（id = 1）.defer（email ）[0] 
>>> x .__ class __.__ mro__ 
（< class'django.contrib.auth.models.User_Deferred_email'>，\ 
< class 'django.contrib.auth.models.User'>，\ 
< class'django.db.models.base.Model'>，< type'object'>）
  
 
An example is better than a thousand words:
   In [3]: User.objects.filter(id=19)[0] == User.objects.filter(id=19)[0]
   Out[3]: True

   In [4]: User.objects.filter(id=19)[0] == User.objects.filter(id=19).defer('email')[0]
   Out[4]: False
Does it work like this on purpose ?

Subquestion: is there any simple way to get a regular model instance from the deferred one ?

EDIT:

It looks like contenttypes framework is patched appropriately: 
http://code.djangoproject.com/changeset/10523

so I would say that the Model._____eq_____() operator shouldn't look like this:
    def __eq__(self, other):
        return isinstance(other, self.__class__) and self._get_pk_val() == other._get_pk_val()
but more like this:
    def __eq__(self, other):
        return ContentType.objects.get_for_model(self) is ContentType.objects.get_for_model(other) and self._get_pk_val() == other._get_pk_val()
This of course causes two DB hits for the first time, but fortunately get_for_model seems to implement cache.
解决方案
Deferred queries return a different class, provided by the deferred_class_factory:
# in db/models/query_utils.py

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
    deferred attributes to a particular instance of the model.
    """
It is basically a proxy, as you can see from the method resolution order:
>>> x = User.objects.filter(id=1).defer("email")[0]
>>> x.__class__.__mro__
(<class 'django.contrib.auth.models.User_Deferred_email'>, \ 
 <class 'django.contrib.auth.models.User'>, \
 <class 'django.db.models.base.Model'>, <type 'object'>)


                        
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