Ruby module that delegates methods to an object

Question

I have an AR object called Run that keeps information of when the run was started, ended, disabled, etc. I am trying to create a class Something and want to make it "runnable". So I created a module called Runnable that takes in a run object on initialization. Now say I mix in this module into a class Something. I want the something object to delegate all the methods defined on run to the run object.

module Runnable
  def initialize(run)
    @run = run
  end

  def self.method_missing(name, *args)
    super unless @run.respond_to?(name)
    @run.send(name, args)
  end
end

class Something
  include Runnable
end

Now if I do:

run = Run.find(123)
run.start_date # works!
something = Something.new(run)
something.start_date # NoMethodError: undefined method `start_date' for #<Something:0x007fb4a276b0c0>

Any idea what I am missing? Is there a better way of doing what I am trying to do?

Cary Swoveland · Accepted Answer · 2018-04-08 05:24:30Z

1

You have two mistakes:

method_missing needs to be an instance method, not a class method
@run.send(name, args) should be @run.send(name, *args). The former results in ArgumentError: wrong number of arguments (given 1, expected 0) when Run#start_date is executed.

class Run
  def start_date
    "Any time now"
  end
end

module Runnable
  def initialize(run)
    @run = run
  end

  def method_missing(name, *args)
    super unless @run.respond_to?(name)
    @run.send(name, *args)
  end
end

class Something
  include Runnable
end

run = Run.new
run.start_date
  #=> "Any time now"
something = Something.new(run)
  #=> #<Something:0x00000002642100 @run=#<Run:0x0000000266d558>>
something.start_date 
  #=> "Any time now"

answered Apr 8, 2018 at 5:24

Cary Swoveland

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Comments

Guillaume · Accepted Answer · 2018-04-07 21:51:05Z

1

It seems to me a SimpleDelegator would achieve what you are looking for here.

answered Apr 7, 2018 at 21:51

Guillaume

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3 Comments

streetsoldier Over a year ago

I thought about that but I can't use that in modules, can I?

Guillaume Over a year ago

Not sure why you want a module here though. You define initialize in it, it seems that this will not be a very flexible module. Why not using inheritance and do class Something < Runnable ?

streetsoldier Over a year ago

Because I have only one shot at inheriting from a class. Once my Something class inherits from Runnable, it cannot inherit from anything else.

streetsoldier · Accepted Answer · 2018-04-07 21:50:11Z

0

Using Forwardable for now

module Runnable
  extend Forwardable
  def_delegator :@run, :start_date, :end_date

  def initialize(run)
    @run = run
  end
end

answered Apr 7, 2018 at 21:50

streetsoldier

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