Rails Metaprogramming:如何在运行时添加实例方法？

Question

我在Rails中定义自己的AR类,其中包括为用户字段0-9动态创建的实例方法.用户字段不直接存储在数据库中,它们将被序列化在一起,因为它们不经常使用.以下是最好的方法吗？备择方案？

从哪里调用添加方法的启动代码？

class Info < ActiveRecord::Base

end

# called from an init file to add the instance methods
parts = []
(0..9).each do |i|
   parts.push "def user_field_#{i}"     # def user_field_0
   parts.push   "get_user_fields && @user_fields[#{i}]"
   parts.push "end"
end

Info.class_eval parts.join

Answer 1

一个很好的方法,特别是如果你可能有超过0..9个用户字段,将使用method_missing:

class Info
  USER_FIELD_METHOD = /^user_field_(\n+)$/
  def method_missing(method, *arg)
    return super unless method =~ USER_FIELD_METHOD
    i = Regexp.last_match[1].to_i
    get_user_fields && @user_fields[i]
  end

  # Useful in 1.9.2, or with backports gem:
  def respond_to_missing?(method, private)  
    super || method =~ USER_FIELD_METHOD
  end
end        

如果您更喜欢定义方法:

10.times do |i|
  Info.class_eval do
    define_method :"user_field_#{i}" do
      get_user_fields && @user_fields[i]
    end
  end
end