Model __eq__ is not symmetric.

[pmcnerthney@…]

The Model test for equal implemented in eq is not symmetric. A non-deferred instance compares equal to a deferred instance, but a deferred instance compares unequal to a non-deferred instance. This is due to the following implementation of eq:

    def __eq__(self, other):
        return isinstance(other, self.__class__ and self._get_pk_val() == other._get_pk_val()

This needs to be changed to:

    def __eq__(self, other):
        return (isinstance(other, self.__class__) or isinstance(self, other.__class__)) and self._get_pk_val() == other._get_pk_val()

Which allows for complete symmetric of comparisons.

[Colin Copeland]

Uploaded a patch with pmcnerthney's fix and an accompanying test.

[Johannes Dollinger]

This patch also changes how __eq__ behaves for inherited models. Given:

class Base(models.Model): pass
class Sub(Base): pass

With the patch sub.base == sub, while currently sub.base != sub.

It also doen't handle the case where both instances have different deferred fields: qs.defer('f1)[0] != qs.defer('f2')[0].

Something like the following could work, but it seems the special-casing should rather happen in deferred_class_factory.

def __eq__(self, other):
    other_model = other.__class__
    if getattr(other_model, '_deferred', False):
        other_model = other_model.mro()[1]

    self_model = self.__class__
    if getattr(self_model, '_deferred', False):
        self_model = self_model.mro()[1]

    return issubclass(other_model, self_model) and self._get_pk_val() == other._get_pk_val()

[Johannes Dollinger]

The patch uses the approach described in my previous comment.

[Johannes Dollinger]

related or duplicate: #16458

[Aymeric Augustin]

#16458 is about inheritance, this one is about deferred instances. They're independent issues with similar symptoms.

[Anssi Kääriäinen]

Replying to aaugustin:

#16458 is about inheritance, this one is about deferred instances. They're independent issues with similar symptoms.

There is a patch in #16458 which solves this ticket.

[Karen Tracey]

#17466 reported this again.

[Tim Graham]

Patch for #16458 is RFC.

[Anssi Kääriäinen]

Fixed in 4668c142dce77c6f29fb75532c1acfa1b2d322ff