From: "Esteban Manchado Velázquez" <zoso@...>
Date: 2005-04-24T03:31:45+09:00
Subject: Re: RoR has_and_belongs_to_many association

Hi Jason,

   Thanks for your answer. Some more details below:

On Sun, Apr 24, 2005 at 02:27:57AM +0900, Jason Foreman wrote:
> [...]
> Many times a many-many relationship like this is really another piece
> of your model that you need to work with, particularly when you have
> additional information associated with the relationship.  You might
> need to add a model C which represents the relationship of a<->b and
> contains those additional attributes.  You haven't given any details
> as to what your A and B are, so I can't give you any insight into what
> the relationship model might be, but consider modelling it as its own
> class.

   Sorry, I thought it wasn't necessary, and as the code is written in Spanish
(for the RoR presentation), I didn't feel like translating :-)

   The main idea is having pupils and classes. You have a pupils table/model
(alumnos) and a classes table/model (cursos). So I have:

   class Alumno <ActiveRecord::Base
      has_and_belongs_to_many :cursos
   end

   class Curso <ActiveRecord::Base
      has_and_belongs_to_many :alumnos
   end

and I have two additional fields in the "alumnos_cursos" table, "reservado"
and "pagado" ("reserved" and "paid"). So, when someone goes and pays some
class, in the code I need to update the "pagado" field in "alumnos_cursos" so
it's not NULL anymore.

> class C < ActiveRecord::Base
>   belongs_to :a;
>   belongs_to :b;
>   # add your foo, bar attributes to this table.
> end

   Hmmm... what about the "id" attribute? I know I can't simply put one, as it
would clobber the alumnos or cursos one when using the h_a_b_t_m relation.

   And, how can i find a particular relation in this table? Should I just
"find_all(:a_id => x, :b_id => y).first", or is there any way to obtain that
object from the has_and_belongs_to_many association (I mean something like
"alumno.cursos.first.relation" to get a AlumnosCursos class)?

   TIA. Regards,

-- 
Esteban Manchado Vel�zquez <zoso@foton.es> - http://www.foton.es
EuropeSwPatentFree - http://EuropeSwPatentFree.hispalinux.es