Define a unique primary key based on 2 columns

Question

I would like to define a unique key for records based on 2 columns : 'id' and 'language'

to let the user submits the following strings : id=1 language=en value=blabla english id=1 language=fr value=blabla french

I tried to use set_primary_key and also add_index but it didn't work ( add_index :words, ["id", "language_id"], :unique => true )

I have the following model :

class Word < ActiveRecord::Base
  belongs_to :dictionnary
  belongs_to :language

  attr_accessible :lang, :rev, :value, :dictionnary_id, :language_id

  validates :value, :uniqueness => true

end  

and this

class Language < ActiveRecord::Base
    has_many :words

    attr_accessible :lang
end

Answer 1

add_index :words, ["id", "language_id"], :unique => true

It should work. Maybe you have already some non-unique data in your db and index can't be created? But (as @Doon noticed it will be redundant since id is always unique). So you need create primary key on two columns.

To define 2 column primary key in rails use:

create_table :words, {:id => false} do |t|
  t.integer :id
  t.integer :language_id
  t.string :value
  t.timestamps
end
execute "ALTER TABLE words ADD PRIMARY KEY (id,language_id);"

And set primary_key in your model with this gem: http://rubygems.org/gems/composite_primary_keys:

class Word < ActiveRecord::Base
    self.primary_keys = :id,:language_id
end