Rails: If Foo has_many :bars, do all :bars need to belong_to a Foo?

Question

I'm working on an app that has a lot of photos in it, which are contributed by users. The photo-sharing aspect is the primary purpose of the app.

Sometimes it's helpful to create featured "Collections" of photos. So, this is easy enough in ActiveRecord: Collection has_many :photos and Photo belongs_to :collection.

The question I have is, for an 'ordinary' photo that doesn't belong to a special collection, should I:

A. Not have it belong to any collection at all. ie: photo.collection == nil

-or-

B. Create a generic "Community Collection" that holds every photo. ie by default photo.collection == 'Community'

I am also considering whether this should be a HABTM relationship.

Feedback would be much appreciated!

Answer 1

It depends on what you're going to do with your uncollected photos, really. If you want the ability to show the uncollected photos with the same action you use to show a collection, then you'll want to have that generic collection. Otherwise, I'd leave it nil; I don't like mixing "seed" data with user data in the same table (which you'd be doing with the "uncollected" collection) - it's too easy to accidentally wipe out your user data when reseeding.

Should it be a HABTM relationship? Again, depends on how you want to use it. If there's a good chance you'll want photos to belong to multiple collections, go for it.

And the things that I've seen mentioned here that I would not do: Adding a 'ptype' attribute to tell singletons from collected photos (waste of DB space - you can already get that info by calling photo.collection_id.nil?); and using polymorphic models (you don't want singleton photos to behave drastically different, I assume, so this is added complexity for little to no benefit).

Named scopes, though, are a great idea, and will be particularly useful if you don't have the "uncollected" collection to find your singletons by:

#Apologies for the Rails 2.3 code - it's what I know...
named_scope :singletons, {:conditions => {:collection_id => nil}}

Or the HABTM version:

named_scope :singletons, lambda {
    {:conditions => ['NOT EXISTS (SELECT * FROM collections_photos WHERE photo_id = ?)', self.id]}
}

Hope this helps!