Validates uniqueness in a one-to-many association

Question

I have an Album model and Track model. I want to make sure that the name of each track on the album they belong to are unique. I tried using this validation in my Track model

validates :name, presence: true, uniqueness: true, scope: :album_id

but get the error: Unknown validator: 'ScopeValidator'

What am I doing wrong?

class Album < ActiveRecord::Base
    has_many :tracks

    accepts_nested_attributes_for :tracks, :reject_if => :all_blank, :allow_destroy => true

    validates :name, presence: true, uniqueness: true
end

class Track < ActiveRecord::Base    
  belongs_to :album

  validates :name, presence: true, uniqueness: true, scope: :album_id
end

Answer 1

You need to scope the uniqueness, not put it as a separate argument.

class Track < ActiveRecord::Base    
  belongs_to :album

  validates :name, presence: true, uniqueness: { scope: :album_id }
end