validates polymorphic fields example...

foo.rb

class Foo < ActiveRecord::Base
  belongs_to :target, polymorphic: true

  # This validation will fail...
  # validates :target, uniqueness: true

  validates :target_type,
            presence: {if: :target_id_present? }
  validates :target_id,
            presence: {if: :target_type_present? },
            uniqueness: {allow_nil: true, scope: :target_type}

  def target_type_present?
    self[:target_type].present?
  end

  def target_id_present?
    self[:target_id].present?
  end
end