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.

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status (accepted, withdrawn, etc.)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 :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 S.'s.

David


David A. Black | [email protected]
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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