In Django, when you have a parent class and multiple child classes that inherit from it you would normally access a child through parentclass.childclass1_set or parentclass.childclass2_set, but what if I don't know the name of the specific child class I want?
Is there a way to get the related objects in the parent->child direction without knowing the child class name?
You can achieve this looking for all the fields in the parent that are an instance of django.db.models.fields.related.RelatedManager. From your example it seems that the child classes you are talking about are not subclasses. Right?
In Python, given a ("new-style") class X, you can get its (direct) subclasses with X.__subclasses__(), which returns a list of class objects. (If you want "further descendants", you'll also have to call __subclasses__ on each of the direct subclasses, etc etc -- if you need help on how to do that effectively in Python, just ask!).
Once you have somehow identified a child class of interest (maybe all of them, if you want instances of all child subclasses, etc), getattr(parentclass,'%s_set' % childclass.__name__) should help (if the child class's name is 'foo', this is just like accessing parentclass.foo_set -- no more, no less). Again, if you need clarification or examples, please ask!
The usual way to do this is to add a ForeignKey to ContentType on the Parent model which stores the content type of the proper "leaf" class. Without this, you may have to do quite a number of queries on child tables to find the instance, depending how large your inheritance tree is. Here's how I did it in one project:
from django.contrib.contenttypes.models import ContentType
class InheritanceCastModel(models.Model):
"""
An abstract base class that provides a ``real_type`` FK to ContentType.
For use in trees of inherited models, to be able to downcast
parent instances to their child types.
"""
real_type = models.ForeignKey(ContentType, editable=False, null=True)
def save(self, *args, **kwargs):
if not self.id:
self.real_type = self._get_real_type()
super(InheritanceCastModel, self).save(*args, **kwargs)
def _get_real_type(self):
return ContentType.objects.get_for_model(type(self))
def cast(self):
return self.real_type.get_object_for_this_type(pk=self.pk)
class Meta:
abstract = True
This is implemented as an abstract base class to make it reusable; you could also put these methods and the FK directly onto the parent class in your particular inheritance hierarchy.
This solution won't work if you aren't able to modify the parent model. In that case you're pretty much stuck checking all the subclasses manually.
Carl's solution is a good one, here's one way to do it manually if there are multiple related child classes:
def get_children(self):
rel_objs = self._meta.get_all_related_objects()
return [getattr(self, x.get_accessor_name()) for x in rel_objs if x.model != type(self)]
It uses a function out of _meta, which is not guaranteed to be stable as django evolves, but it does the trick and can be used on-the-fly if need be.
It turns out that what I really needed was this:
Model inheritance with content type and inheritance-aware manager
That has worked perfectly for me. Thanks to everyone else, though. I learned a lot just reading your answers!