How do you validate uniqueness of a pair of ids in Ruby on Rails?

Question

Suppose the following DB migration in Ruby:

     create_table :question_votes do |t|       t.integer :user_id       t.integer :question_id       t.integer :vote        t.timestamps     end 

Suppose further that I wish the rows in the DB contain unique (user_id, question_id) pairs. What is the right dust to put in the model to accomplish that?

validates_uniqueness_of :user_id, :question_id

seems to simply make rows unique by user id, and unique by question id, instead of unique by the pair.

Answer 1

validates_uniqueness_of :user_id, :scope => [:question_id] 

if you needed to include another column (or more), you can add that to the scope as well. Example:

validates_uniqueness_of :user_id, :scope => [:question_id, :some_third_column]