Rails - Jun 2006 - id column for join table... kosher?

I''ve got two tables, bookmarks & tags. Using a  
has_and_belongs_to_many association, I can do lookups using a join  
table called bookmarks_tags. Two questions:

1. Can I have a migration for my join tables? Rails seems to "know"  
about join tables implicitly from the associations, but if I do a  
rake migrate the join tables won''t be built. I''ve been
creating them
using SQL/by hand.

2. Is there anything wrong with having an ''id'' column on my
join
table? Rails expects:
bookmark_id & tag_id

...but what if I want to reference individual joins? Can I do:
bookmark_id & tag_id & id

Does this mess anything up? The Agile book says don''t do it, articles  
on the web say it''s OK. Who to believe?

Thanks for any/all advice!
-Jason

1.   migration
   create_table :bookmarks_tags, :id=>false do |t|
     t.column :bookmark_id. :integer
     t.column :tag_id, :integer
   end

2. No ids on join tables.  However, see has_many :through     That lets you
do a has_and_belongs_to_many using a model as the middle piece instead of a
join table. Then you *can* have an id on that table. However, I would advise
against this unless you have additional fields on the join

2.b  Looks like you''re doing tagging. Have a look ath the
acts_as_taggable
plugin.  http://wiki.rubyonrails.com/rails/pages/Acts+As+Taggable+Plugin(You
want the plugin, not the Gem!)

2 c.  I don''t get your question about individual joins.  With a HABTM
relationship, you just do

    @bookmark = Bookmark.find(1)
    @tags = @bookmark.tags

    @bookmark.tags << Tag.create :name => "ajax"

   ...
   etc

So what would you be doing with joins?

Hope some of that helps you.


On 6/22/06, Jason Frankovitz <jason@seethroo.us>
wrote:
>
> I''ve got two tables, bookmarks & tags. Using a
> has_and_belongs_to_many association, I can do lookups using a join
> table called bookmarks_tags. Two questions:
>
> 1. Can I have a migration for my join tables? Rails seems to
"know"
> about join tables implicitly from the associations, but if I do a
> rake migrate the join tables won''t be built. I''ve been
creating them
> using SQL/by hand.
>
> 2. Is there anything wrong with having an ''id'' column on
my join
> table? Rails expects:
> bookmark_id & tag_id
>
> ...but what if I want to reference individual joins? Can I do:
> bookmark_id & tag_id & id
>
> Does this mess anything up? The Agile book says don''t do it,
articles
> on the web say it''s OK. Who to believe?
>
> Thanks for any/all advice!
> -Jason
>
>
> _______________________________________________
> Rails mailing list
> Rails@lists.rubyonrails.org
> http://lists.rubyonrails.org/mailman/listinfo/rails
>
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>
> Message: 4
> Date: Thu, 22 Jun 2006 14:10:16 -0500
> From: "Brian Hogan" <bphogan@gmail.com>
> Subject: Re: [Rails] id column for join table... kosher?
> To: rails@lists.rubyonrails.org
> Message-ID:
> 	<95a868100606221210s1d67d95aq3cbaedf497181a7a@mail.gmail.com>
> Content-Type: text/plain; charset="iso-8859-1"
>
> 1.   migration
>    create_table :bookmarks_tags, :id=>false do |t|
>      t.column :bookmark_id. :integer
>      t.column :tag_id, :integer
>    end
Cool, thanks.
>
> 2. No ids on join tables.  However, see has_many :through     That  
> lets you
> do a has_and_belongs_to_many using a model as the middle piece  
> instead of a
> join table. Then you *can* have an id on that table. However, I  
> would advise
> against this unless you have additional fields on the join
>
I do have add''l fields for the join table so has_many :through looks  
like the way to go. I know this is a new feature of Rails. Are there  
any good tutorials about implementing it?

> 2.b  Looks like you''re doing tagging. Have a look ath the  
> acts_as_taggable
> plugin.  http://wiki.rubyonrails.com/rails/pages/Acts+As+Taggable 
> +Plugin(You
> want the plugin, not the Gem!)
>
THanks, I saw that. What I''m doing right now won''t work with
it.

> 2 c.  I don''t get your question about individual joins.  With a
HABTM
> relationship, you just do
>
>     @bookmark = Bookmark.find(1)
>     @tags = @bookmark.tags
>
>     @bookmark.tags << Tag.create :name => "ajax"
>
>    ...
>    etc
>
> So what would you be doing with joins?
I think what I want to do is slightly unusual. I want to have a join  
table that has three ids being matched up, not just two. I want to  
identify a series of three-way relationships ("TWR''s") and
attach
meta-data to an individual TWR. For example, I want to combine users,  
bookmarks, and tags in one join table, then have additional info that  
records when a TWR record was created, what browser they used to  
create the TWR, etc. I guess you could think of it as an "Event"  
model involving the three id''s and meta-data.

Can has_many :through handle a three-way? (err, ahem)

Thanks in advance!		
-Jason