Rails - Sep 2006 - has_and_belongs_to_many or new class?

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?
In general, when is better to use each one?
If is a recurring question, can you point me to a good paper?
Thanks.

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Hi --

On Sat, 30 Sep 2006, Floren wrote:
>
> 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     status (accepted, withdrawn, etc.)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 :-)  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

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                   David A. Black |
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Author of "Ruby for Rails"   [1] | Ruby/Rails training &
consultancy [3]
DABlog (DAB''s Weblog)        [2] | Co-director, Ruby Central, Inc.  
[4]
[1] http://www.manning.com/black | [3] http://www.rubypowerandlight.com
[2] http://dablog.rubypal.com    | [4] http://www.rubycentral.org

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