validate parent model (See related posts)

class Book < ActiveRecord::Base
  has_many :pages
end

class Pages < ActiveRecord::Base
  belongs_to :book
  
  validates_presence_of :page_id # this is not enough, anything can go in this field

  # we check if the parent object is valid or not
  validate do |page|
   if page.blank?
     unless page.book(true)
       errors.add(:page_id, "must be a valid Book id")
     end
   end
  end

end