ruby-on-rails - 不能在前置模块中使用实例变量

Question

我希望能够将我的模块包含在 ActiveRecord::Base 中，以便 has_folder_attachments 方法可用于我的 Rails AR 类。

我这样做是为了扩展原始模块的功能以支持 AR Hook ；但是变量 @physical_path 和 @dice 都是 nil，我不明白为什么。

module FolderAttachments
  module ClassMethods
    def has_folder_attachments(physical_path, excludes: [])
      @physical_path = physical_path
      super
    end 
  end  

  def self.prepended(base)
    class << base
      prepend ClassMethods
    end  
  end  

  attr_reader :physical_path
end

module ActiveRecord
  class Base
    prepend FolderAttachments

    attr_reader :dice

    # This should run after the module method
    def self.has_folder_attachments(*args)
      @dice = true
    end
  end
end

class Damned < ActiveRecord::Base
  has_folder_attachments :for_real
end

damn = Damned.new
puts damn.physical_path # => nil
puts damn.dice          # => nil

Answer 1

在使用这两个变量时，您正在混合实例和(元)类上下文。这两个变量都在类上下文中运行的方法中设置它们的值(更准确地说是在 metaclass 的上下文中)。因此，您无法在实例上下文中访问这些变量(及其 attr_reader)。

要使 attr_reader 正常工作，您必须将它们移动到类上下文并从那里访问它们:

module FolderAttachments
  module ClassMethods
    ...
    attr_reader :physical_path
  end
end

module ActiveRecord
  class Base
    ...
    class << self
      attr_reader :dice
    end
  end
end

damn = Damned.new
damn.class.physical_path # => :for_real
damn.class.dice          # => true

或者您还可以添加委托(delegate)给类级读取器的实例级读取器，以便您也可以在实例上下文中访问它们:

module FolderAttachments
  module ClassMethods
    ...
    attr_reader :physical_path
  end

  def physical_path
    self.class.physical_path
  end
end

module ActiveRecord
  class Base
    ...
    class << self
      attr_reader :dice
    end

    def dice
     self.class.dice
    end
  end
end

damn = Damned.new
damn.physical_path # => :for_real
damn.dice          # => true