has_and_belongs_to_many or new class?

Hi --

Hello, I have groups and events. To each event I want to invite some groups. Groups has many inviting_events (events that invited a group) and each event has_many groups invited. I want to access through @group.inviting_events, @group.future_events (the same but with a condition) and @event.invited_groups. Is it better to create a new model that has_one group and has_one event or to use a has_and_belongs_to_many relation?

I would say what you want is a new model that belongs_to group and event -- namely, an invitation:

   Event    Group    Invitation      event_id      group_id      status (accepted, withdrawn, etc.)      created_at

You can then do:

   class Event < AR::Base      has_many :invitations      has_many :groups, :through => "invitations"

and so forth, using conditions to fine-tune the various collections.

In general, when is better to use each one?

I have a feeling the answer you'll hear most is: habtm is quasi-obsolete :slight_smile: The main thing is that having a third model means that you can express things more richly. In my example, Invitation has a status and a created_at timestamp; those are just to illustrate the fact that Invitation is a model in its own right, and can have its own characteristics. A habtm link table is just a place to stash foreign keys; it can't store any further information about the association.

The main thing that has stopped me from thinking of the :through technique as a superset of habtm is the fact that with habtm you can do this:

   e = Event.find(m)    g = Group.find(n)    e.groups << g

However, there is now work being done on implementing << for the :through technique. See http://blog.hasmanythrough.com/articles/2006/08/19/magic-join-model-creation

If is a recurring question, can you point me to a good paper?

http://blog.hasmanythrough.com/articles/2006/04/20/many-to-many-dance-off and generally a lot of the stuff on that blog, which is Josh Susser's.

David