Define a unique primary key based on 2 columns

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