30

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

Andrei
  • 10,918
  • 12
  • 76
  • 110
  • If you really want to avoid `events`, you'll need to implement the query to the `Event` table manually, as suggested below. I'd still prefer having `events` as a GenericRelation and then use `self.events.first()` in the `event` property. Also, this would more easily allow you to remove the unique constraint, in the future. – alexcasalboni Jul 13 '17 at 15:53

2 Answers2

21

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

Community
  • 1
  • 1
Timmy O'Mahony
  • 53,000
  • 18
  • 155
  • 177
  • Its works well, but it's not give the ability to do: `Action.objects.select_related('event')` – ramusus May 24 '13 at 12:58
  • 5
    I don't like this solution -> a `GenericRelation` looks much better, even if you need to use `self.events.get()` or `self.events.first()` – alexcasalboni Jul 13 '17 at 15:50
  • Same as the comments above. What you had originally with `GenericRelation` is much better. – s-block Feb 27 '18 at 10:13
11

GenericRelation is a class for representing a Generic Many to One and adding a first_event property allow to represent a Generic One to One.

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 = generic.GenericRelation(Event)

    @property
    def first_event(self):
        return self.events.first()

In addition, I recommend using .first() inside Action1.first_event instead of .get() due to, it returning None if the event doesn't exist. .get() raises an exception if the event doesn't exist and this behavior can be unwanted.

Leonardo Ramírez
  • 520
  • 6
  • 8