AR Uniqueness of Multiple Columns

cols_unique.rb

# https://stackoverflow.com/questions/34424154/rails-validate-uniqueness-of-two-columns-together

# answer 1
class AddUniqueIndexToReleases < ActiveRecord::Migration
  def change
    add_index :releases, [:country, :medium], unique: true
  end
end

class Release < ActiveRecord::Base
  validates :country, uniqueness: { scope: :medium }
end

# answer 2
validates :country, uniqueness: { scope: [:medium, :another_medium] }