Question

from myapp.models import MyModel
from cPickle import *

tmp = MyModel.objects.all()[:1]
print(loads(dumps(t, -1)) == t)
#Output is "False"

In my case pickled query result differs from unpickled. I already read here:
https://docs.djangoproject.com/en/dev/ref/models/querysets/#pickling-querysets
that such operations are actually allowed. So – what am I doing wrong?

upd #1: Tried cPickle and regular Pickle – got ‘False’ from both

upd #2: Possible resolution – converting QuerySet to Python list with list(). Found it while reading these: https://docs.djangoproject.com/en/dev/ref/models/querysets/#when-querysets-are-evaluated

Answer 1

The problem is that you are trying to compare two querysets and querysets doesn’t have a __cmp__ method defined.

So, you can compare a queryset with itself and you will get this:

>> tmp == tmp
True

This is because, as there are no __cmp__ method, == evaluates True if both objects have the same identity (the same memory address). You can read it from here

So, when you do this:

>> loads(dumps(tmp, -1)) == tmp
False

you will get a False as a result because objects have different memory addresses. If you convert queries into a “comparable” object, you can get the behaviour you want. Try with this:

>> set(loads(dumps(tmp, -1))) == set(tmp)
True

Hope it helps!