Sure. OOM relationships can typically be broken down along two dimensions: cardinality and directionality.
cardinality: DIRECTIONALITY ---------------------------------- One-to-one: ONE-WAY, BI-WAY One-to-many: ONE-WAY, BI-WAY Many-to-many: ONE-WAY, BI-WAY The cardinality is pretty obvious, it's just one-to-one, one-to-many, or many-to-many, as we'd all expect. Django assumes that all relationships are bi-directional, meaning that if there's a relationship between A and B then you can access that relationship from A and you can also access it from B. However, in some cases (like the one I originally asked the question about), it's useful to have a one-way relationship, such that you can access B from A, but not the other way around. In this case, the directionality would be one-way. I'm making up all the terminology here, but hopefully the idea comes across. Django assumes bidirectionality[1], but it would be nice to be able to make uni-directional relationships for those cases, like Photo, where the photo can be owned by multiple different models in a one-to-many way. To keep the models clean my plan is to use your many-to-many suggestion, but it does result in an unnecessary table and associated join, which is a bummer but tolerable. Thanks again, Mike [1] From http://www.djangoproject.com/documentation/db-api/#how-are-the-backward-relationships-possible: "Other object-relational mappers require you to define relationships on both sides. The Django developers believe this is a violation of the DRY (Don’t Repeat Yourself) principle, so Django only requires you to define the relationship on one end." On May 13, 12:03 pm, "Scott Moonen" <[EMAIL PROTECTED]> wrote: > Michael, can you elaborate on what you mean by "forcing bi-directional > relationships"? > > The ManyToManyField approach really is, I think, the "right" way to do it. > If you think about it, a hypothetical ManyToOneField in your case would work > almost exactly like the ManyToManyField. The join table would be structured > exactly the same, except it would have an additional UNIQUE(photo_id) > qualifier. > > -- Scott > > > > On Tue, May 13, 2008 at 2:56 PM, Michael Burton <[EMAIL PROTECTED]> wrote: > > > Thanks much, Scott. They both seem a bit hacky, but it gives me > > something to work with anyway. > > > I recognize the motivation for forcing bi-directional relationships in > > Django was done to keep things DRY[1], but does anyone know if there's > > been any discussion about maybe relaxing this constraint? Seems a > > little restrictive, and I don't think most other web frameworks go > > this route for that very reason... > > > Mike > > > [1] > >http://www.djangoproject.com/documentation/db-api/#how-are-the-backwa... > > > On May 13, 11:32 am, "Scott Moonen" <[EMAIL PROTECTED]> wrote: > > > Michael, you have two alternatives: > > > > 1. Create ManyToManyField fields in the UserProfile and Place models, > > > pointing to Photo. "ManyToManyField" may seem a bit odd since you > > really > > > have a many-to-one relation, but it will work as you expect, creating > > a join > > > table connecting each pair of models. > > > 2. Create two ForeignKey fields in Photo, one to UserProfile and one > > > to Photo, with null=True. Yes, this is a bit ugly. :) > > > > -- Scott > > > > On Tue, May 13, 2008 at 2:24 PM, Michael Burton <[EMAIL PROTECTED]> > > wrote: > > > > > I have some Places and I have some Users in my database. I'd like to > > > > be able to associate some Photos with each. > > > > > class Photo(models.Model): > > > > # a model that represents a photo > > > > > class UserProfile(models.Model): > > > > # has a list of Photos > > > > > class Place(models.Model): > > > > # has a list of Photos > > > > > Normally, if i were using another ORM framework, I would make my Place > > > > have a list of photos, and I'd make my UserProfile have a list of > > > > photos, and I'd leave my Photo model alone. However, the Django way > > > > of doing things requires that I put a ForeignKey into my Photo model > > > > to establish the one-to-many. > > > > > The problem is, sometimes Photo's ForeignKey will point to a > > > > UserProfile and sometimes to an Place. How can I have both my > > > > UserProfile and Place models point to Photos? > > > > > Thanks in advance, > > > > Mike > > > > --http://scott.andstuff.org/|http://truthadorned.org/<http://scott.andstuff.org/%7Chttp://truthadorned.org/> > > --http://scott.andstuff.org/|http://truthadorned.org/ --~--~---------~--~----~------------~-------~--~----~ You received this message because you are subscribed to the Google Groups "Django users" group. To post to this group, send email to django-users@googlegroups.com To unsubscribe from this group, send email to [EMAIL PROTECTED] For more options, visit this group at http://groups.google.com/group/django-users?hl=en -~----------~----~----~----~------~----~------~--~---
