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我正在使用rails-settings gem，我试图了解如何将函数添加到 ActiveRecord 类（我正在为纸牌游戏构建自己的库），我注意到这个 gem 使用了 Meta-programming 之一将函数添加到 ActiveRecord::Base 类的技术（我远离 ruby​​ 的元编程大师，但我正在努力学习它）

module RailsSettings
  class Railtie < Rails::Railtie

    initializer 'rails_settings.initialize', :after => :after_initialize do
      Railtie.extend_active_record
    end

  end

  class Railtie
    def self.extend_active_record
      ActiveRecord::Base.class_eval do
        def self.has_settings
          class_eval do
            def settings
              RailsSettings::ScopedSettings.for_thing(self)
            end

            scope :with_settings, :joins => "JOIN settings ON (settings.thing_id = #{self.table_name}.#{self.primary_key} AND
                                                               settings.thing_type = '#{self.base_class.name}')",
                                  :select => "DISTINCT #{self.table_name}.*"

            scope :with_settings_for, lambda { |var| { :joins => "JOIN settings ON (settings.thing_id = #{self.table_name}.#{self.primary_key} AND
                                                                                    settings.thing_type = '#{self.base_class.name}') AND
                                                                                    settings.var = '#{var}'" } }

            scope :without_settings, :joins => "LEFT JOIN settings ON (settings.thing_id = #{self.table_name}.#{self.primary_key} AND
                                                                       settings.thing_type = '#{self.base_class.name}')",
                                     :conditions => 'settings.id IS NULL'

            scope :without_settings_for, lambda { |var| { :joins => "LEFT JOIN settings ON (settings.thing_id = #{self.table_name}.#{self.primary_key} AND
                                                                                            settings.thing_type = '#{self.base_class.name}') AND
                                                                                            settings.var = '#{var}'",
                                                          :conditions => 'settings.id IS NULL' } }
          end
        end
      end
    end

  end
end

我不明白的是为什么他在 ActiveRecord::Base 上使用 class_eval，如果他只是打开 ActiveRecord::Base 类并定义函数不是更容易吗？特别是块中没有任何动态（我所说的动态是指当您对包含变量的字符串执行 class_eval 或 instance_eval 时）

像这样的东西：

module ActiveRecord
  class Base
    def self.has_settings
      class_eval do
        def settings
          RailsSettings::ScopedSettings.for_thing(self)
        end

        scope :with_settings, :joins => "JOIN settings ON (settings.thing_id = #{self.table_name}.#{self.primary_key} AND
                                                           settings.thing_type = '#{self.base_class.name}')",
                              :select => "DISTINCT #{self.table_name}.*"

        scope :with_settings_for, lambda { |var| { :joins => "JOIN settings ON (settings.thing_id = #{self.table_name}.#{self.primary_key} AND
                                                                                settings.thing_type = '#{self.base_class.name}') AND
                                                                                settings.var = '#{var}'" } }

        scope :without_settings, :joins => "LEFT JOIN settings ON (settings.thing_id = #{self.table_name}.#{self.primary_key} AND
                                                                   settings.thing_type = '#{self.base_class.name}')",
                                 :conditions => 'settings.id IS NULL'

        scope :without_settings_for, lambda { |var| { :joins => "LEFT JOIN settings ON (settings.thing_id = #{self.table_name}.#{self.primary_key} AND
                                                                                        settings.thing_type = '#{self.base_class.name}') AND
                                                                                        settings.var = '#{var}'",
                                                      :conditions => 'settings.id IS NULL' } }
      end
    end
  end
end

我知道第二个class_eval（在def设置之前）是在每个“has_settings”的类上动态定义函数对吗？同样的问题，我认为他可以使用“def self.settings”而不是“class_eval .... def settings”，不是吗？

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1 回答 1

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rails-settings代码所做的被认为是好的做法：它只会在明确要求这样做时才会与第三方模块混淆。这样，您还可以保持命名空间整齐分离，并且所有代码都保留在模块中。

于 2011-01-16T11:44:40.540 回答