Hi Kip, Thanks for your help with this. :)
MTI is meant to be Multi-Table Inheritance, sorry. I copied the reference from other Django related docs. :) About using that generic_relations example, it's hard to explain without diagrams but I'll give it a try... Based on that example: Animals/Vegetables/Minerals has-a (composition) tag If I were to use that as a base with the Generic Foreign Key in enclosure: Animals (Zebra, Snake) has-a (composition) enclosure But what I want to achieve is: Enclosure has-a (composition) Animal (Zebra, Snake) And that's kind of why I did the models that way. I'm starting to think that Django may not be able to support this type of design without multi-table inheritance which I don't feel is a particularly clean way of implementing this. I also don't know if Django's binding of models to a relational database model will lend itself to this very well. :) I'll keep looking anyway. Thanks for the help! Shay On Aug 21, 7:18 pm, Kip Parker <[EMAIL PROTECTED]> wrote: > Though I'm not sure what MTI is (Google suggest "Massage Training > Institute", but that can't be right), what you're trying to do seems > well within Django's capabilities. > > Generic relations allow you to create foreign keys to any other > object, so you'd have a Generic Foreign Key in your enclosure, then > that could contain any kind of animal (or tree, car or building for > that matter). Have another look > here:http://www.djangoproject.com/documentation/models/generic_relations/, > it's so close to what you're doing that I think you can steal most of > the code you need. > > Alll the best, > > Kip > > On Aug 21, 6:40 am, Shay <[EMAIL PROTECTED]> wrote: > > > Hi, > > > Thanks for the tip, I found Generic Relations and GenericForeignKey > > shortly after I posted this and you confirmed I was looking in the > > right place. :) > > > It seemed a better idea to make Animalabstractsince I don't want it > > to be able to be instantiated on its own. ZooEnclosure should only be > > able to accept anything that is a type of Animal but not Animal itself > > since it contains no valuable information in isolation of its > > subclasses. Is there a way of preventing this without making the > > classabstract? > > > Also are there any scalability issues with using MTI for this type of > > design (Particularly on large tables)? > > > With respect to implementing that design as a generic relation (sort > > of based onhttp://www.djangoproject.com/documentation/contenttypes/ > > andhttp://www.webmonkey.com/tutorial/Build_a_Microblog_with_Django), > > I'm still a little fuzzy on this, but would the implementation be > > something to the effect of: > > > class Animal(models.Model): > > content_type = models.ForeignKey(ContentType) > > object_id = models.PositiveIntegerField() > > content_object = generic.GenericForeignKey('content_type', > > 'object_id') > > ... > > > class Meta: > > abstract= True > > > class Zebra(Animal): > > /* Inherits the content_type, object_id, content_object attributes > > from Animal */ > > ... > > > class Snake(Animal): > > ... > > > class ZooEnclosure(models.Model): > > enclosureName = models.CharField(max_length=50) > > > And then when ZooEnclosure is instantiated (form or otherwise), > > there's either hook or some signal event which will trigger the > > creation of the instance of Animal. > > > Am I on the right track or completely off? > > > Also since ZooEnclosure could contain any type of animal, I can't > > recreate a reverse GenericRelation here for inhabitedBy. Creating a > > GenericRelation to Animal would, I'm guessing, leave me with the same > > problem as before because I set it asabstract(and the same other > > problem of unflattened table hierarchy if I didn't set it asabstract). Is > > it possible to specify this in the model? > > > If these are elementary questions, my apologies, I'm still trying to > > get my head around how all of this hangs together. :) > > > Thanks, > > Shay > > > On Aug 20, 5:21 pm, Kip Parker <[EMAIL PROTECTED]> wrote: > > > > Maybe have a look at generic > > > relations?http://www.djangoproject.com/documentation/models/generic_relations/ > > > > You could also use multi-table inheritance rather thanabstract > > > classes, then use the parent class Animal as the key in ZooEnclosure. > > >Abstractclasses can't exist on their own, which I expect is why you > > > got the NoneType errors. > > > > Kip. > > > > On Aug 20, 3:57 am, Shay <[EMAIL PROTECTED]> wrote: > > > > > Hi, > > > > > I'm rather new to Django, so please bear with me. :P > > > > > I've been trying to figure out how to implement the following in > > > > models: > > > > > class: Animal (abstractclass) > > > > ... > > > > > class: Zebra (extends Animal) > > > > ... > > > > > class: Snake (extends Animal) > > > > ... > > > > > class: ZooEnclosure > > > > String enclosureName > > > > Animal inhabitedBy > > > > > So: > > > > - This is a very trivial example > > > > - Zebra and Snake are subclasses of theabstractclass Animal > > > > - ZooEnclosure is composed of some sort of Animal where I could > > > > feasibly use a Zebra or a Snake > > > > > Creating a new ZooEnclosure for every animal is not an option because > > > > there're quite a few and could quite possibly result in a hundred > > > > different classes just to change the animal. > > > > > I've looked at the MTI and ABC type documentation (and Google) but I'm > > > > still not clear on how best to implement this in Django Models. > > > > > Oh and when I have attempted to leave Animalabstract, it gives me > > > > NoneType errors. > > > > > I'm not sure what the best way to tackle this problem is, any > > > > suggestions? > > --~--~---------~--~----~------------~-------~--~----~ You received this message because you are subscribed to the Google Groups "Django users" group. 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