I started composing a reply to your response, and then realized that I had erroneously constructed a mental one-to-one model of the relationship rather than one-to-many. You're absolutely right, what I was trying to do doesn't make any sense. Whoops :)
Mike On May 13, 2:28 pm, "Scott Moonen" <[EMAIL PROTECTED]> wrote: > Michael, I think I understand what you're getting at, given your comment > referring to the "unnecessary table and associated join". I think you want > to be able to say "ManyToOneField" in Place and have it reach back into the > Photo model and insert a NULLable "place_id" column. That's problematic for > several reasons: > > 1. The Photo model is no longer self-contained. You can't look at the > model and infer the table layout. So, for example, you can't just syncdb > on > the Photo model's application and get the table you want. Without > knowledge > of other applications (potentially in other projects!) you don't know what > the table will look like. > 2. It produces inelegant tables at best (I'm too lazy to confirm > whether it actually violates any normalization rules). If you have many > models referring to Photo in this way, it will have equally many columns, > most of which will be NULL in any case. > > If you want it to work this way, I think it is best that you are explicit > about the presence of the foreign keys and code ForeignKey(x, null=True) > within the Photo model. But if you want the "directionality" to go the way > you indicate, I do think it is correct and more elegant to have the > intermediate join table. > > -- Scott > > > > On Tue, May 13, 2008 at 3:41 PM, Michael Burton <[EMAIL PROTECTED]> wrote: > > > 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-backwa... > > : > > "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/%7Chttp://truthadorned.org/> > > > > --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 -~----------~----~----~----~------~----~------~--~---
