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

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.

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.

When you are creating a new record, that doesn't work because the id of your parent model doesn't exist still at moment of validations.

This should to work for you.

class B < ActiveRecord::Base
  has_many :ab
  has_many :a, :through => :ab
end

class AB < ActiveRecord::Base
  belongs_to :b
  belongs_to :a
end

class A < ActiveRecord::Base
  has_many :ab
  has_many :b, :through => :ab

  after_validation :validate_uniqueness_b

  private
  def validate_uniqueness_b
    b_ids = ab.map(&:b_id)
    unless b_ids.uniq.length.eql? b_ids.length
      errors.add(:db, message: "no repeat b's")
    end
  end
end

In the above code I get all b_id of collection of parameters, then compare if the length between the unique values and obtained b_id are equals.
If are equals means that there are not repeat b_id.

Note: don't forget to add unique in your database's columns.