ruby inheritance vs mixins

Question

I just read about this topic in The Well-Grounded Rubyist (great book, by the way). The author does a better job of explaining than I would so I'll quote him:

---

No single rule or formula always results in the right design. But it’s useful to keep a couple of considerations in mind when you’re making class-versus-module decisions:

• Modules don’t have instances. It follows that entities or things are generally best modeled in classes, and characteristics or properties of entities or things are best encapsulated in modules. Correspondingly, as noted in section 4.1.1, class names tend to be nouns, whereas module names are often adjectives (Stack versus Stacklike).

• A class can have only one superclass, but it can mix in as many modules as it wants. If you’re using inheritance, give priority to creating a sensible superclass/subclass relationship. Don’t use up a class’s one and only superclass relationship to endow the class with what might turn out to be just one of several sets of characteristics.

Summing up these rules in one example, here is what you should not do:

module Vehicle 
... 
class SelfPropelling 
... 
class Truck < SelfPropelling 
  include Vehicle 
... 

Rather, you should do this:

module SelfPropelling 
... 
class Vehicle 
  include SelfPropelling 
... 
class Truck < Vehicle 
... 

The second version models the entities and properties much more neatly. Truck descends from Vehicle (which makes sense), whereas SelfPropelling is a characteristic of vehicles (at least, all those we care about in this model of the world)—a characteristic that is passed on to trucks by virtue of Truck being a descendant, or specialized form, of Vehicle.

I think mixins are a great idea, but there's another problem here that nobody has mentioned: namespace collisions. Consider:

module A
  HELLO = "hi"
  def sayhi
    puts HELLO
  end
end

module B
  HELLO = "you stink"
  def sayhi
    puts HELLO
  end
end

class C
  include A
  include B
end

c = C.new
c.sayhi

Which one wins? In Ruby, it turns out the be the latter, module B, because you included it after module A. Now, it's easy to avoid this problem: make sure all of module A and module B's constants and methods are in unlikely namespaces. The problem is that the compiler doesn't warn you at all when collisions happen.

I argue that this behavior does not scale to large teams of programmers-- you shouldn't assume that the person implementing class C knows about every name in scope. Ruby will even let you override a constant or method of a different type. I'm not sure that could ever be considered correct behavior.

My take: Modules are for sharing behavior, while classes are for modeling relationships between objects. You technically could just make everything an instance of Object and mix in whatever modules you want to get the desired set of behaviors, but that would be a poor, haphazard and rather unreadable design.

The answer to your question is largely contextual. Distilling pubb's observation, the choice is primarily driven by the domain under consideration.

And yes, ActiveRecord should have been included rather than extended by a subclass. Another ORM - datamapper - precisely achieves that!

I like Andy Gaskell's answer very much - just wanted to add that yes, ActiveRecord should not use inheritance, but rather include a module to add the behavior (mostly persistence) to a model/class. ActiveRecord is simply using the wrong paradigm.

For the same reason, I very much like MongoId over MongoMapper, because it leaves the developer the chance to use inheritance as a way of modelling something meaningful in the problem domain.

It's sad that pretty much nobody in the Rails community is using "Ruby inheritance" the way it's supposed to be used - to define class hierarchies, not just to add behavior.

The best way I understand mixins are as virtual classes. Mixins are "virtual classes" that have been injected in a class's or module's ancestor chain.

When we use "include" and pass it a module, it adds the module to the ancestor chain right before the class that we are inheriting from:

class Parent
end 

module M
end

class Child < Parent
  include M
end

Child.ancestors
 => [Child, M, Parent, Object ...

Every object in Ruby also has a singleton class. Methods added to this singleton class can be directly called on the object and so they act as "class" methods. When we use "extend" on an object and pass the object a module, we are adding the methods of the module to the singleton class of the object:

module M
  def m
    puts 'm'
  end
end

class Test
end

Test.extend M
Test.m

We can access the singleton class with the singleton_class method:

Test.singleton_class.ancestors
 => [#<Class:Test>, M, #<Class:Object>, ...

Ruby provides some hooks for modules when they are being mixed into classes/modules. included is a hook method provided by Ruby which gets called whenever you include a module in some module or class. Just like included, there is an associated extended hook for extend. It will be called when a module is extended by another module or class.

module M
  def self.included(target)
    puts "included into #{target}"
  end

  def self.extended(target)
    puts "extended into #{target}"
  end
end

class MyClass
  include M
end

class MyClass2
  extend M
end

This creates an interesting pattern that developers could use:

module M
  def self.included(target)
    target.send(:include, InstanceMethods)
    target.extend ClassMethods
    target.class_eval do
      a_class_method
    end
  end

  module InstanceMethods
    def an_instance_method
    end
  end

  module ClassMethods
    def a_class_method
      puts "a_class_method called"
    end
  end
end

class MyClass
  include M
  # a_class_method called
end

As you can see, this single module is adding instance methods, "class" methods, and acting directly on the target class (calling a_class_method() in this case).

ActiveSupport::Concern encapsulates this pattern. Here's the same module rewritten to use ActiveSupport::Concern:

module M
  extend ActiveSupport::Concern

  included do
    a_class_method
  end

  def an_instance_method
  end

  module ClassMethods
    def a_class_method
      puts "a_class_method called"
    end
  end
end