Rails - Dec 2008 - Checking if an association already exists between two records

Hey,

I''m creating an app which has two models - bands and users. A
relationship between users and bands in many to many.

However, when I''m adding an existing to user to a band (ie. updating
the bands_users table) how can i check if the relationship between the
user and the band already exists? For example, if a user with ID of 3
was already a member of a band with ID 5, and then somebody tried to
make this connection again - I want to output an error message. Is
there some special validation I can place in a bands_users.rb that
validated the uniqueness of the entire record?

Thanks,
  -ger
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You can use the scopt attribute of validates_uniqueness_of
But this would require, that the intermediate table between
users/bands has it''s own model

in this model (BandsUsers)
validates_uniqueness_of :user_id, :scope => band_id


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On Mon, Dec 1, 2008 at 11:07 AM, Gearóid O''Ceallaigh
<gearoid.k-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org>
wrote:
>
> Hey,
>
> I''m creating an app which has two models - bands and users. A
> relationship between users and bands in many to many.
>
> However, when I''m adding an existing to user to a band (ie.
updating
> the bands_users table) how can i check if the relationship between the
> user and the band already exists? For example, if a user with ID of 3
> was already a member of a band with ID 5, and then somebody tried to
> make this connection again - I want to output an error message. Is
> there some special validation I can place in a bands_users.rb that
> validated the uniqueness of the entire record?
There is a :uniq option that will ignore duplicates:

  class Band < ActiveRecord::Base
    has_and_belongs_to_many :users, :uniq => true
  end

This won''t raise an error, but it will prevent duplicate associations.
 Hope that Helps

Brandon
-- 
--------------------------------------------------------------------------------
Training by Collective Idea: Ruby on Rails training in a vacation setting
http://training.collectiveidea.com — San Antonio, TX — Jan 20-23

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I guess you have your reasons but why not ignore the fact that the
relationship already existed? Does it really matter to let the user
know?

Pepe

On Dec 1, 12:29 pm, "Brandon Keepers"
<bran...-NYlJC1mBhkRg9hUCZPvPmw@public.gmane.org>
wrote:
> On Mon, Dec 1, 2008 at 11:07 AM, Gearóid O''Ceallaigh
>
> <gearoi...-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org> wrote:
>
> > Hey,
>
> > I''m creating an app which has two models - bands and users. A
> > relationship between users and bands in many to many.
>
> > However, when I''m adding an existing to user to a band (ie.
updating
> > the bands_users table) how can i check if the relationship between the
> > user and the band already exists? For example, if a user with ID of 3
> > was already a member of a band with ID 5, and then somebody tried to
> > make this connection again - I want to output an error message. Is
> > there some special validation I can place in a bands_users.rb that
> > validated the uniqueness of the entire record?
>
> There is a :uniq option that will ignore duplicates:
>
>   class Band < ActiveRecord::Base
>     has_and_belongs_to_many :users, :uniq => true
>   end
>
> This won''t raise an error, but it will prevent duplicate
associations.
>  Hope that Helps
>
> Brandon
> --
>
--------------------------------------------------------------------------------
> Training by Collective Idea: Ruby on Rails training in a vacation
settinghttp://training.collectiveidea.com— San Antonio, TX — Jan 20-23
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