Django proxy model and ForeignKey

Question

How to make entry.category to be instance of CategoryProxy? See code for details:

class Category(models.Model): pass

class Entry(models.Model):
    category = models.ForeignKey(Category)

class EntryProxy(Entry):
    class Meta:
        proxy = True

class CategoryProxy(Category):
    class Meta:
        proxy = True

entry = EntryProxy.objects.get(pk=1)
entry.category # !!! I want CategoryProxy instance here

Cast from Category to CategoryProxy is ok too, but I am not very familiar with ORM internals to properly copy internal state...

EDIT. Reason: I added method to CategoryProxy and want to use him:

EntryProxy.objects.get(pk=1).category.method_at_category_proxy()

EDIT 2. Currently I implemented it like this:

EntryProxy._meta.get_field_by_name('category')[0].rel.to = CategoryProxy

but it looks terrible...

Answer 1

To switch from a model class to a proxy class without hitting the database:

class EntryProxy(Entry):
    @property
    def category(self):
        new_inst = EntryProxy()
        new_inst.__dict__ = super(EntryProxy, self).category.__dict__
        return new_inst

edit: the snippet above seems not working on django 1.4.

Since django 1.4, I take all value fields manually like this:

class EntryProxy(Entry):
    @property
    def category(self):
        category = super(EntryProxy, self).category
        new_inst = EntryProxy()
        for attr in [f.attname for f in category.__class__._meta.fields] + ['_state']:
            setattr(new_inst, attr, getattr(category, attr))
        return new_inst

To switch from a queryset to a child proxy class without hitting database:

class CategoryProxy(Category):
    @property
    def entry_set(self):
        qs = super(CategoryProxy, self).entry_set
        qs.model = EntryProxy
        return qs

Answer 2

This is an open Django issue: #10961 (Allow users to override forward and reverse relationships on proxy models with ForeignKey fields)

You can work around it by resetting the fields in question after you define the proxy models:

EntryProxy.add_to_class('category', CategoryProxy)