Model __eq__ does not compare correctly between normal objects and deferred objects

[Raphaël Hertzog <hertzog@…>]

Enrico Zini <enrico@…> found a problem and reported it to the Debian Bug Tracking System: ​http://bugs.debian.org/653177

I'll paste his report below:

---

I noticed that there is a problem with the default model object __eq__
implementation when working with deferred objects.

The default implementation should be something like:

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

Now, given a simple model...

        class Package(models.Model):
                name = models.TextField(null=False, unique=True)
                # Lots of other fields

...this code will show the problem:

        # Get a normal object
        p = Package.objects.get(name="debtags")
        # Get a deferred object
        dp = Package.objects.filter(name="debtags").only("name")[0]
        print object.__repr__(p), hash(p)
        print object.__repr__(dp), hash(dp)
        # This assert succeeds
        assert hash(p) == hash(dp)
        # This assert fails
        assert p == dp

Its output is:

        <Package: debtags> 11
        <Package_Deferred_ldesc_popcon_sdesc_source_version: debtags> 11

It looks like the isinstance bit of the default __eq__ model is failing
here.

This causes suprising behaviour:

        allpkgs = set(Package.objects.all().only("name"))
        # This assert fails
        assert Package.objects.get(name="debtags") in allpkgs

[Karen Tracey]

This looks to be the same problem as described in #11892.