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

ruby-on-rails validation model unique-constraint
dfrankow picture dfrankow · May 29, 2009 · Viewed 32.3k times · Source

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

Demi picture Demi · May 29, 2009 
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]

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