ruby强大的 define_method，有闭包的能力！

闭包的例子一般是在说ruby的块，特片是：访问本地变量和绑定运行上下文（很强大的两点）

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class Metadata  attr_accessor :relation_nameendmodule Accessor             def build(name, metadata)    p name, metadata, metadata.relation_name  end  module ClassMethods    def getter(name, metadata)      tap do        define_method(name) do           build(          name,          metadata          )        end      end    end  end        end   class Doc  include Accessor  extend Accessor::ClassMethodsendmeta = Metadata.newmeta.relation_name = 'embeds_one'Doc.getter("ss_car", meta)# p Doc.public_instance_methodsDoc.new.ss_carmeta2 = Metadata.newmeta2.relation_name = 'ref_one'Doc.getter("ss_bus", meta2)Doc.new.ss_bus                          

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输出：

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"ss_car"
#<Metadata:0x00000100846f38 @relation_name="embeds_one">
"embeds_one"
"ss_bus"
#<Metadata:0x00000100846128 @relation_name="ref_one">
"ref_one"

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