Django QuerySet .defer() problem - bug or feature?

Question

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.

Answer 1

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'>)