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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