Rails - Aug 2005 - ActiveRecord Question : Populating Extra Columns in Join Tables

Hey everyone,

I''ve been tooling around with Rails for a little while now and I just
came across my first question that I haven''t been able to find an
answer to :).

I''ve got two models, Playlist and Track, that describe my music
library. Tracks contain the information about songs, and Playlists are
composed of any number of Tracks (in addition to any information
unique to a playlist, such as its name). So there''s also a
playlists_tracks table that can join the two tables together. Nothing
fancy.

So to add a track to a playlist, I can do the following (minus any
error checking)...

track = Track.find(@params[''track''].to_i)
playlist = Playlist.find(@params[''id''].to_i)
track.playlists << playlist

The code listed above will create a record in the playlists_tracks
table, as desired. The only problem is that the playlists_tracks table
contains more information than just the playlist_id column and the
track_id column. For example, there''s an ''enabled''
column that
indicates whether or not this track is enabled (ie: should be played)
or not. And there may be more information added further on down the
road...

How can I populate the extra column(s) in the playlists_tracks table
when creating a new record as shown above without resorting to
manually writing SQL? Is it possible?

Thanks for any help you can provide. 

dave

On 8/23/05, dave miller
<davidfmiller-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org>
wrote:
> Hey everyone,
> 
> I''ve been tooling around with Rails for a little while now and I
just
> came across my first question that I haven''t been able to find an
> answer to :).
> 
> I''ve got two models, Playlist and Track, that describe my music
> library. Tracks contain the information about songs, and Playlists are
> composed of any number of Tracks (in addition to any information
> unique to a playlist, such as its name). So there''s also a
> playlists_tracks table that can join the two tables together. Nothing
> fancy.
> 
> So to add a track to a playlist, I can do the following (minus any
> error checking)...
> 
> track = Track.find(@params[''track''].to_i)
> playlist = Playlist.find(@params[''id''].to_i)
> track.playlists << playlist
> 
> The code listed above will create a record in the playlists_tracks
> table, as desired. The only problem is that the playlists_tracks table
> contains more information than just the playlist_id column and the
> track_id column. For example, there''s an
''enabled'' column that
> indicates whether or not this track is enabled (ie: should be played)
> or not. And there may be more information added further on down the
> road...
> 
> How can I populate the extra column(s) in the playlists_tracks table
> when creating a new record as shown above without resorting to
> manually writing SQL? Is it possible?
> 
> Thanks for any help you can provide.
> 
> dave
Looking at the docs:
http://rails.rubyonrails.com/classes/ActiveRecord/Associations/ClassMethods.html#M000430,
you can use push_with_attributes.

@developer.projects.push_with_attributes(@project, :started_at => Time.now)

<% @developer.projects.each do |project| -%>
<%= @developer.name %> started <%= project.title %> on <%=
project.started_at %>
<% end -%>

Yes, it fills the object with the extra attribute automatically.  It
seems like this worked both ways:

@project.started_at = Time.now
@developer.projects << @project

But, the docs don''t mention it.  I can''t guarantee results
from my flaky memory.

A word of warning, Rails will have absolutely no clue what type those
extra fields are.  Since that join table is not a model, rails never
grabs the table info, so just assumes every field is a string.  If
that third table needs more stuff (validations, callbacks, a primary
key, etc), then it''s better off being its own model with a has_many
relationship to the other two models.

-- 
rick
http://techno-weenie.net

> If that third table needs more stuff (validations, callbacks, a primary
> key, etc), then it''s better off being its own model with a
has_many
> relationship to the other two models.
Actually, wouldn''t it be a belongs_to relationship to the other two
model classes?  And the original two model classes would each have a
has_many to the third one.

Ken

> Actually, wouldn''t it be a belongs_to relationship to the other
two
> model classes?  And the original two model classes would each have a
> has_many to the third one.
Ah right you are.  I had the associations backwards.  


-- 
rick
http://techno-weenie.net

On 8/23/05, Rick Olson
<technoweenie-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org>
wrote:
> On 8/23/05, dave miller
<davidfmiller-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org> wrote:
> > Hey everyone,
> >
> > I''ve been tooling around with Rails for a little while now
and I just
> > came across my first question that I haven''t been able to
find an
> > answer to :).
> >
> > I''ve got two models, Playlist and Track, that describe my
music
> > library. Tracks contain the information about songs, and Playlists are
> > composed of any number of Tracks (in addition to any information
> > unique to a playlist, such as its name). So there''s also a
> > playlists_tracks table that can join the two tables together. Nothing
> > fancy.
> >
> > So to add a track to a playlist, I can do the following (minus any
> > error checking)...
> >
> > track = Track.find(@params[''track''].to_i)
> > playlist = Playlist.find(@params[''id''].to_i)
> > track.playlists << playlist
> >
> > The code listed above will create a record in the playlists_tracks
> > table, as desired. The only problem is that the playlists_tracks table
> > contains more information than just the playlist_id column and the
> > track_id column. For example, there''s an
''enabled'' column that
> > indicates whether or not this track is enabled (ie: should be played)
> > or not. And there may be more information added further on down the
> > road...
> >
> > How can I populate the extra column(s) in the playlists_tracks table
> > when creating a new record as shown above without resorting to
> > manually writing SQL? Is it possible?
> >
> > Thanks for any help you can provide.
> >
> > dave
> 
> Looking at the docs:
>
http://rails.rubyonrails.com/classes/ActiveRecord/Associations/ClassMethods.html#M000430,
> you can use push_with_attributes.
> 
> @developer.projects.push_with_attributes(@project, :started_at =>
Time.now)
> 
> <% @developer.projects.each do |project| -%>
> <%= @developer.name %> started <%= project.title %> on <%=
project.started_at %>
> <% end -%>
> 
> Yes, it fills the object with the extra attribute automatically.  It
> seems like this worked both ways:
> 
> @project.started_at = Time.now
> @developer.projects << @project
> 
> But, the docs don''t mention it.  I can''t guarantee
results from my flaky memory.
> 
> A word of warning, Rails will have absolutely no clue what type those
> extra fields are.  Since that join table is not a model, rails never
> grabs the table info, so just assumes every field is a string.  If
> that third table needs more stuff (validations, callbacks, a primary
> key, etc), then it''s better off being its own model with a
has_many
> relationship to the other two models.
> 
> --
> rick
> http://techno-weenie.net
> _______________________________________________
> Rails mailing list
> Rails-1W37MKcQCpIf0INCOvqR/iCwEArCW2h5@public.gmane.org
> http://lists.rubyonrails.org/mailman/listinfo/rails
> 

-- 
dave