无法在预审算模块中使用实例变量

Question

我希望能够将我的模块包含在ActivereCord ::基础中，以便我的Rails AR类可用has_folder_attachments方法。

我正在这样做以扩展原始模块的功能以支持AR钩子；但是，变量@physical_path和@dice都是零，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

使用两个变量时，您正在混合实例和(META(类上下文。这两个变量均以在类上下文中运行的方法(更准确地说是在Metaclass的上下文中(设置的值。因此，您无法在实例上下文中访问这些变量(及其attr_reader s(。

要使attr_reader s工作，您必须将它们移至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

或者您还可以添加将其委派给类级读者委派的实例级读者，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