I realise everyone's been busy with getting 1.3 ready, but doesn't anyone
have thoughts on this? It's been two weeks ...
Thanks
Craig
On Thursday, February 17, 2011 11:08:57 PM UTC+13, Craig de Stigter wrote:
>
> Hi folks
>
> Ever since Django started supporting various types of model inheritance
> I've wondered why it lacks the kind that I would find most useful: python
> behaviour differentiated based on the value of a field.
>
> I'll explain with an example. Here's what I'd like to do:
>
> class Datasource(models.Model):
> type = models.ModelTypeField()
> uri = models.CharField(max_length=256)
>
> # common behavior in the superclass
> def __repr__(self):
> return u'<%s: %s>' % (self.__class__.__name__, self.uri)
>
> class HttpDatasource(Datasource):
> # custom behaviour in the subclasses
> def get_filename(self):
> return self.uri.rsplit("/", 1)[-1]
>
> class ZipfileDatasource(Datasource):
> def get_filename(self):
> files = zipfile.list(self.uri)
> return files[0].rsplit('/', 1)[-1]
>
> >>> zip = ZipfileDatasource.objects.create(uri="/path/to/foo.zip")
> >>> uri = UriDatasource.objects.create(uri="http://example.com/foo.txt";)
>
> >>> Datasource.objects.all()
> [<ZipfileDatasource: /path/to/foo.zip>, <UriDatasource:
> http://example.com/foo.txt>]
>
> >>>ZipfileDatasource.objects.all()
> [<ZipfileDatasource: /path/to/foo.zip>]
>
> >>> Datasource.objects.all().values_list('type', flat=True)
> [u'myapp.models.ZipfileDatasource', u'myapp.models.UriDatasource']
>
>
> These are quite similar to proxy models, but vary in their queryset
> behaviour - the generic Datasource queryset has mixed types and the concrete
> querysets are always filtered by type.
>
> This is far more useful than proxy models, since the concrete types of each
> table are known. It's also better than making an abstract model and
> subclassing it, because now all the objects are in the same table and you
> can iterate over them all at once if you want. Adding more types is easy,
> since there are no schema changes (with abstract models you'd have to add a
> new table for each type).
>
> It's possible that proxy models could be extended to do this without
> breaking existing code. I'm not sure yet.
>
> I'm thinking of diving into this. Does anyone have any suggestions? Or,
> perhaps there's a reason why this hasn't been done previously that you more
> experienced gurus could illuminate?
>
> Thanks in advance
>
> Craig de Stigter
> Maintainer of django-mptt and full time dev at koordinates.com
>
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