dictsort by obj and by obj_id gives varying results

[apollovy@…]

Hello there.

Story:

I've got a model:

class MyModel(Model):
 obj = ForeignKey(SomeOtherModel)

I've got a view:

class MyView(ListView):
 def get_context_data(self):
  return {'objects': MyModel.objects.all()}

Then I've got a template:

{% regroup objects|dictsort:'obj_id' by obj %}
{% regroup objects|dictsort:'obj' by obj %}

And I've got data:

MyModel: [{id: 1, obj: 1},{id: 2, obj: 1},{id: 3, obj: 2},{id: 4, obj: 2},{id: 5, obj: 1}]
SomeOtherModel: [{id: 1},{id: 2}]

Dictsort in first case works well and predictable.
Second case - no way to figure out. One time objects got grouped in 2 groups, as expected. The other time - in 3 groups. The other time - in 4(!) groups.

Guess this behaviour is not meant.

[Baptiste Mispelon]

Hi,

MyModel.obj_id will give you back an integer so sorting a list according to this attribute is straightforward and does what you expect.

However, MyModel.obj will give you back an instance of SomeOtherModel, which causes the behaviour you're describing, because comparing two model instances is somehow undefined (technically, it's very well defined but it most likely never does what you want).

The technical reason behind that is that django's models don't implement a __cmp__ method so when comparing them, python falls back to comparing their id (id being the python builtin function, not the primary key of the model) which is not consistent.

See http://docs.python.org/2/reference/datamodel.html#object.__cmp__ for more details.

Note that this is not applicable to python3 where comparing two model instances together will fail with a TypeError.

[Baptiste Mispelon]

I'm going to mark this as invalid, since this is not a bug but only a python2 "quirk".