Rails数组中单个对象的多个实例?

听起来您有多对多关系,一个项目可能在多个发票中(一次以上),一个发票有多个项目。

class Invoice < ActiveRecord::Base
  has_many :invoice_items
  has_many :items, :through => :invoice_items
end

class InvoiceItem < ActiveRecord::Base
  belongs_to :invoice
  belongs_to :item
end