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

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]

If using mysql, you can do it in the database using a unique index. It's something like:

add_index :question_votes, [:question_id, :user_id], :unique => true

This is going to raise an exception when you try to save a doubled-up combination of question_id/user_id, so you'll have to experiment and figure out which exception to catch and handle.

The best way is to use both, since rails isn't 100% reliable when uniqueness validation come thru.

You can use:

  validates :user_id, uniqueness: { scope: :question_id }

and to be 100% on the safe side, add this validation on your db (MySQL ex)

  add_index :question_votes, [:user_id, :question_id], unique: true

and then you can handle in your controller using:

  rescue ActiveRecord::RecordNotUnique

So now you are 100% secure that you won't have a duplicated value :)

From RailsGuides. validates works too:

class QuestionVote < ActiveRecord::Base
  validates :user_id, :uniqueness => { :scope => :question_id }
end

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.