How to make entry.category to be instance of CategoryProxy? See code for details:
class Category(models.Model): pass
class Entry(models.Model):
category
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))
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)
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.
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.
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
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