A product page has a price, line items, orders and customers.

product_page.rb

class ProductPage < Page
  
  cattr_accessor :allowed_children
  @@allowed_children = [Page,ArchivePage]
  
  layout 'product'
  
  field 'price'
  
  part 'body'
  part 'images'
  
  after_create :set_price
  
  def sku    
    url.gsub('\/','-')
  end
  
  def description
    part('body').content if has_part('body')
  end
  
  def price
    field('price').content.to_f unless field('price').nil?
  end
  
  def line_items
    LineItem.all(:joins => :fields, :conditions => [ 'page_fields.name = ? AND page_fields.content = ?', 'product_id', self.id.to_s])
  end
  
  def customers
    line_items.map(&:created_by).flatten.compact.uniq
  end
  
  def orders
    line_items.map(&:parent).flatten.compact.uniq
  end
  
  def to_json(*attrs)
    super self.class.params
  end
  
  def to_xml(*attrs)
    super self.class.params
  end
  
  class << self
    
    # Returns attributes attached to the product
    def attrs
      [ :id, :title, :created_at, :updated_at ]
    end
    
    # Returns methods with usefuly information
    def methds
      [ :price ]
    end
    
    # Returns a custom hash of attributes on the product
    def params
      { :only  => self.attrs, :methods => self.methds }
    end
    
  end
  
  private
  
  def set_price
    self.price = (price || 0.00)
  end
  
end

Product = ProductPage