Dear all,
I am playing around with model inheritance introduced in django 1.0.
In particular, I am trying to achieve something that might be called
dynamic-casts (with reference to C++). Consider the following example
type hierarchy:
MediaObject
|
+-- ImageObject
| |
| +-- PngObject
| +-- GifObject
|
+-- AudioObject
|
+-- Mp3Object
+-- WavObject
I want to retreive a set of MediaObjetcs from the database and treat
them with respect to their actual type. E.g. I want to display them
using specialized templates, like
{% for object in objects %}
{% include object.template %}
{% endfor %}
(or using some more sophisticated template dispatcher).
The problem I am running into is the following: when retrieving
MediaObjects from the database, e.g. via
obj = get_objet_or_404(MediaObject,id=1)
they become upcasted to actual MediaObjects no matter what their
type has been on object creation, and I cannot pass the correct type
-- get_object_or_404(Mp3Object,id=1) -- nor access the child object as
the attribute obj.mp3object -- simply because the actual type is
unknown
on retrieval.
I tried to use the contenttype application and hold a reference to the
original objects type in MediaObject:
class MediaObject(models.Model) :
final_type = models.ForeignKey(ContentType)
def __init__(self,*args) :
super(MediaObject,self).__init__(*args)
self.final_type =
ContentType.objects.get_for_model(type(self))
class AudioObject(MediaObject) :
pass
so that I could dynamically upcast objects to their original type with
obj =
get_object_or_404(MediaObject,id=1).final_type.get_object_for_this_type(id=1)
It does not work at all. For some reason, final_type also gets
upcasted
to MediaObject?!
Is there any (possibly elegant) way to achieve this functionality?
Thanks!
- harold -
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