Django proxy model and ForeignKey

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
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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...

---

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

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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)

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None of the current solutions (including the accepted one) work with Django 2.0.

Building on Matt Schinckel's work on overriding proxy model relations, here's a solution that will work with Django 2.0 and 2.1.

---

This question already has an accepted answer, but wanted to post this for anyone who may come searching.

You can patch the model at runtime with the new field so that relations work as expected. A full example can be seen here - https://gist.github.com/carymrobbins/8721082

from django.db.models.fields.related import ReverseSingleRelatedObjectDescriptor

def override_model_field(model, field, field_name, column_name):
    """Force override a field in a Django Model.
    Usage: override_model_field(
        MyModel, models.ForeignKey(OtherModel), 'other', 'other_id')
    :type model: django.db.models.base.ModelBase
    :type field: django.db.models.fields.Field
    :type field_name: basestring
    :type column_name: basestring
    """
    field.name = field_name
    field.attname = column_name
    for i, f in enumerate(model._meta.fields):
        if f.name == field_name:
            model._meta.fields[i] = field
            break
    else:
        raise TypeError('Model {!r} does not have a field {!r}.'
                        .format(model, field_name))
    model.add_to_class(field_name,
                       ReverseSingleRelatedObjectDescriptor(field))

---

You can add proxy model to ForeignKey:

class Entry(models.Model):
    category = models.ForeignKey(CategoryProxy, on_delete=models.CASCADE)

and now:

entry = EntryProxy.objects.get(pk=1)
entry.category # CategoryProxy instance

---

Define a property category on EntryProxy that looks up the CategoryProxy by its id:

class EntryProxy(Entry):
    @property
    def category(self):
        cid = super(EntryProxy, self).category.id
        return CategoryProxy.objects.get(id=cid)

    class Meta:
        proxy = True

---

Adapting Bernd Petersohn's answer slightly, we then have:

class EntryProxy(Entry):
    @property
    def category(self):
        return CategoryProxy.objects.get(id=self.category_id)

This ought to be more economical with the database. For added improvements you could set a private attribute (self._category) the first time the method is called, then return that all subsequent times.