#9317: Return content type of child class when parent passed to
ContentType.objects.get_for_model
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Reporter: svetlyak40wt | Owner: nobody
Status: closed | Milestone:
Component: Contrib apps | Version: 1.0
Resolution: wontfix | Keywords: contenttype
Stage: Unreviewed | Has_patch: 1
Needs_docs: 0 | Needs_tests: 0
Needs_better_patch: 0 |
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Changes (by mtredinnick):
* status: new => closed
* needs_better_patch: => 0
* resolution: => wontfix
* needs_tests: => 0
* needs_docs: => 0
Comment:
I'm a strong -1 on this for reasons that have already been discussed a lot
on both django-dev and django-users. This is related to why querying a
parent model shouldn't automatically cast to the descendent instances
(which aren't necessarily children, but could be grand-children or even
further removed).
Firstly, `get_for_model()` returns the content type for the model you pass
in. You're trying to change that to return the content type for some
other, related model (the instance you pass in *is* a `Parent`, it's not
down-cast to be one of the child types, so the content type is the content
type of `Parent`). For everybody, not just your call.
Secondly, this is a very expensive operation. If a parent model has 3
children, each of which have 3 children of their own, you've just executed
''at least'' 12 database lookups (possibly more if there are other
`OneToOneFields` on the models -- you're heavily misusing
`_collect_related()` here). Thirdly, there's the implementation issue that
nothing outside the Model class should be using a method starting with an
underscore.
There's probably a useful utility set of functions that somebody could
build (as a third-party application initially) that does things like
descendent content types and down-casting to the right child,
automatically looks for a field such as child_type, etc. But those
utilities shouldn't initially be targeted at Django's core, since they
don't need to modify anything there. Keep in mind the multi-layer, multi-
child inheritance situation, since these things start to scale very poorly
once you build up any type of significant hierarchy.
So thanks for writing a patch, but this isn't appropriate as a regular use
thing in core. There are too many drawbacks.
--
Ticket URL: <http://code.djangoproject.com/ticket/9317#comment:1>
Django <http://code.djangoproject.com/>
The Web framework for perfectionists with deadlines.
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