Rails - Feb 2006 - [Req] polymorphic associations howto

Hi people,

I''m looking into polymorphic associations as a way to cleanup some 
messy parts of my code. I think they will allow me to do exectly what 
I need, but since there''s little to none written on this subject,
I''d be
delighted if someone could shed some light on this. Especially what 
the different options and needed table structures need to be.

Thanks,

- Fabien

On 10 Feb 2006, at 10:35, Fabien Franzen wrote:
> Hi people,
>
> I''m looking into polymorphic associations as a way to cleanup some
> messy parts of my code. I think they will allow me to do exectly what
> I need, but since there''s little to none written on this subject,
> I''d be
> delighted if someone could shed some light on this. Especially what
> the different options and needed table structures need to be.
Chad Fowler has a rails recipe in the works talking about polymorphic  
associations. It should be in one of the next beta versions of the  
book. I can highly recommend it, it goes there where the Agile book  
stops.

Best regards

Peter De Berdt

On 2/10/06, Peter De Berdt <peter.de.berdt@pandora.be>
wrote:
>
> On 10 Feb 2006, at 10:35, Fabien Franzen wrote:
>
> > Hi people,
> >
> > I''m looking into polymorphic associations as a way to cleanup
some
> > messy parts of my code. I think they will allow me to do exectly what
> > I need, but since there''s little to none written on this
subject,
> > I''d be
> > delighted if someone could shed some light on this. Especially what
> > the different options and needed table structures need to be.
>
> Chad Fowler has a rails recipe in the works talking about polymorphic
> associations. It should be in one of the next beta versions of the
> book. I can highly recommend it, it goes there where the Agile book
> stops.
>
> Best regards
>
> Peter De Berdt
Polymorphic joins are kind of like an inverted STI setup.  Before, I''d
have to use STI to join a single Asset model to multiple models
(Articles, Books, Albums, for example).

class Asset < ActiveRecord::Base
end

class ArticleAsset < ActiveRecord::Base
  belongs_to :article, :foreign_key => ''object_id''
end

class Article < ActiveRecord::Base
  has_many :assets, :class_name => ''ArticleAsset''
end

Using polymorphic joins gets rid of the redundant *Asset models:

class Asset < ActiveRecord::Base
  # describes a generic ''attachable'' class
  # uses attachable_id as foreign key, and attachable_type
  belongs_to :attachable, :polymorphic => true
end

class Article < ActiveRecord::Base
  has_many :assets, :as => :attachable
end

Don''t let this stop you from getting the Rails Recipe book.  I
can''t
recommend it enough.  And, I''m quite sure Chad''s explanation
will be
much better ;)

--
Rick Olson
http://techno-weenie.net

> Polymorphic joins are kind of like an inverted STI setup.  Before,
I''d
> have to use STI to join a single Asset model to multiple models
> (Articles, Books, Albums, for example).
> 
> class Asset < ActiveRecord::Base
> end
> 
> class ArticleAsset < ActiveRecord::Base
>   belongs_to :article, :foreign_key => ''object_id''
> end
> 
> class Article < ActiveRecord::Base
>   has_many :assets, :class_name => ''ArticleAsset''
> end
Thanks Rick,

This is exactly the kind of info I''m looking for. I also used STI to
solve
this kind of thing before; it''s the ''mess'' I was
talking about ;-)

Looking forward to see it revealed further in the beta book!

- Fabien