Lacking Documentation on Custom Related Fields Pre-Save Override

[s.shanabrook@…]

The process to create a custom field that inherits a OneToOneField or a ForeignKey (and I assume a ManyToManyField as well) and overrides the pre_save method, is not clear in the documentation. ​The section that outlines how to override the pre_save method says that the attribute name is self.attname, when in fact on a related field, self.attname is the model's id field for the related object.

So in order to actually modify the related object and then save it, you must do something like this:

class CustomRelatedField(ForeignKey):
    def pre_save(self, model_instance, add):
        related_object = getattr(model_instance, self.name)
        # Do stuff with object, custom saving

        id = getattr(related_object, 'id', None)
        setattr(model_instance, self.attname, id)
        setattr(model_instance, self.name, redirect)
        return id

[Tim Graham]

Not sure what to do here as attname isn't referenced in the linked doc (maybe it's since been removed). This seems in the realm of private API anyway.