Chain of Responsibility and alias_method problems in Ruby

Question

I'm trying to implement the chain of responsibility pattern in Ruby and ActiveRecord for a polymorphic object. I'm having a few problems.

• Sometimes I get an error that a method is not defined when I try to alias_method it, I think this is because the class isn't loaded or something so I explicity do a send to get the method
• I get a bunch of infinite chains where the aliased function (original_method) calls method which calls original_method. I'm wondering if this is because when you alias a method that's already been overwritten, you're in essence making "original_method" a copy of the aliased method.
• I'm currently working around this by having a function like "chained" return a sub-class of Setting with all the defined methods but curious why there were so many problems with alias_method right in the class.

Here's an example:

class Hospital
  has_one :setting, :as => :settable
  belongs_to :administrative_area

  def next_link
    adminstrative_area
  end

  def usable_setting
    setting ? setting : next_link.usable_setting
  end
end

Then, I have a Setting object:

class Setting < ActiveRecord::Base

belongs_to :settable, :polymorphic => true

def chained
  %w(api_key active_days).each do |method|

    # this is here because otherwise the method isn't defined,
    # it's almost as while it's going up, the metaclass doesn't have the columns
    # until it loads, probably will be fixed if we cache classes
    self.send method.to_sym

    (class << self; self; end).class_eval do

      define_method method do |*args|
        alias_method "original_#{method}", method
        my_setting = send("original_#{method}")
        if my_setting.nil? or my_setting.empty?
          settable.next_link.usable_setting.chained.send(method)
        else
          return my_setting
        end
      end
    end
  end
  self
end
end

Answer 1

You seem to be overcomplicating. Seems that you're trying to see if api_key and active_days exists, and if not, get it from somewhere else.

Here's the right way to do it, assuming that api_key and active_days are columns in your table:

class Setting < ActiveRecord::Base

  belongs_to :settable, :polymorphic => true

  def api_key
    super || settable.next_link.usable_setting.api_key
  end

  def active_days
    super || settable.next_link.usable_setting.active_days
  end
end

You can refactor it a bit to keep clarity and remove duplication.

class Setting < ActiveRecord::Base

  belongs_to :settable, :polymorphic => true

  def api_key
    super || next_usable_setting.api_key
  end

  def active_days
    super || next_usable_setting.active_days
  end

  private
  def next_usable_setting
    settable.next_link.usable_setting
  end
end

So in this case notice — if you have api_key/active_days available, it will get returned. Otehrwise, it will go fetch usable_setting from next_link. If that one has api_key/active_days, it will get returned, otherwise it will fetch usable_setting from next_link. Etc.