gistfile1.rb

module Primaryable
  def self.included(base)
    base.before_create  :create_primary
    base.before_destroy :destroy_primary
    base.before_save    :save_primary
    base.before_update  :update_primary
    
    base.named_scope :primary_first, { :order => "`primary` desc" }
    base.named_scope :primary_last, { :order => "`primary` asc" }
  end

  def siblings
    primaryable_siblings
  end
  
private

  def create_primary
    self.primary = true if siblings.empty?
  end

  def destroy_primary
    siblings.first.update_attribute :primary, true if primary? && siblings.present?
  end

  def original_primary
    new_record? ? nil : self.class.find(id).primary
  end

  def primary_changed?
    primary != original_primary
  end

  def primaryable_siblings
    super
  end

  def save_primary
    self.class.update_all "`primary` = 0", "id in (#{sibling_ids.join(', ')})" if primary? && siblings.present?
  end
  
  def secondary?
    !primary?
  end
  
  def sibling_ids
    siblings.collect(&:id)
  end
  
  def update_primary
    if siblings.empty?
      self.primary = true
    else
      siblings.first.update_attribute :primary, true if primary_changed? && secondary?
    end
  end

end