Generic one-to-one relation in Django

Question

I need to set up one-to-one relation which must also be generic. May be you can advice me a better design. So far I came up to the following models

class Event(models.Model):     # skip event related fields...     content_type      = models.ForeignKey(ContentType)     object_id         = models.PositiveIntegerField()     content_object    = generic.GenericForeignKey('content_type', 'object_id')      class Meta:         unique_together   = ('content_type', 'object_id')  class Action1(models.Model):     # skip action1 related fields...     events = generic.GenericRelation(Event, content_type_field='content_type', object_id_field='object_id')      @property     def event(self):         return self.events.get() # <<<<<< Is this reasonable?  class Action2(models.Model):... 

In Django Admin in event list I want to collect all actions, and from there I want go to admin pages for actions. Is it possible to avoid creating event property in the action models? Is there a better solution? It would be nice to combine the field events and the property event in a single definition. The project I am working with uses Django 1.1

Answer 1

I recently came across this problem. What you have done is fine, but you can generalise it a little bit more by creating a mixin that reverses the relationship transparently:

class Event(models.Model):     content_type      = models.ForeignKey(ContentType)     object_id         = models.PositiveIntegerField()     content_object    = generic.GenericForeignKey('content_type', 'object_id')      class Meta:         unique_together   = ('content_type', 'object_id')  class EventMixin(object):      @property      def get_event(self):          ctype = ContentType.objects.get_for_model(self.__class__)          try:              event = Event.objects.get(content_type__pk = ctype.id, object_id=self.id)          except:             return None           return event  class Action1(EventMixin, models.Model):     # Don't need to mess up the models fields (make sure the mixing it placed before models.Model)     ... 

and

action = Action1.object.get(id=1) event = action.get_event 

You might want to add caching to the reverse relationship too

Answer 2

Using the name events_relation for the GenericRelation inside the 'Action1' class is an elegant way to clearly differentiate event from events.

Inside Action1.event I recommend using .first() due to it returning None if the object doesn't exist. .get() raises an exception if the object doesn't exist and this behavior can be unwanted.

class Event(models.Model):     content_type      = models.ForeignKey(ContentType)     object_id         = models.PositiveIntegerField()     content_object    = generic.GenericForeignKey('content_type', 'object_id')      class Meta:         unique_together   = ('content_type', 'object_id') # Important  class Action1(models.Model):     events_relation = generic.GenericRelation(Event)      @property     def event(self):         # Return the object in exists         # else None          return self.events_relation.first()