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

user_34935 0
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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.

validation unique-constraint model ruby-on-rails
6 Answers
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]
user_679850

Except for writing your own validate method, the best you could do with validates_uniqueness_of is this:

validates_uniqueness_of :user_id, :scope => "question_id"

This will check that the user_id is unique within all rows with the same question_id as the record you are attempting to insert.

But that's not what you want.

I believe you're looking for the combination of :user_id and :question_id to be unique across the database.

In that case you need to do two things:

  1. Write your own validate method.
  2. Create a constraint in the database because there's still a chance that your app will process two records at the same time.
user_1121770
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