#20105: dictsort by obj and by obj_id gives varying results
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Reporter: apollovy@… | Owner: nobody
Type: Uncategorized | Status: new
Component: Template system | Version: 1.5
Severity: Normal | Resolution:
Keywords: | Triage Stage:
dictsort,sorting,grouping,regroup,templatetag| Unreviewed
Has patch: 0 | Needs documentation: 0
Needs tests: 0 | Patch needs improvement: 0
Easy pickings: 0 | UI/UX: 0
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Changes (by bmispelon):
* cc: bmispelon@… (added)
* needs_better_patch: => 0
* needs_tests: => 0
* needs_docs: => 0
Comment:
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`.
--
Ticket URL: <https://code.djangoproject.com/ticket/20105#comment:1>
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