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多个外键指向一个模型，比较极端的例子：
http://stackoverflow.com/questions/307581/rails-model-has-many-with-multiple-foreign-keys
class Person < ActiveRecord::Base
belongs_to :father, :class_name => 'Person'
belongs_to :mother, :class_name => 'Person'
has_many :children_of_father, :class_name => ' ...