Question

It is known that in Ruby, class methods get inherited:

class P
  def self.mm; puts 'abc' end
end
class Q < P; end
Q.mm # works

However, it comes as a surprise to me that it does not work with mixins:

module M
  def self.mm; puts 'mixin' end
end
class N; include M end
M.mm # works
N.mm # does not work!

I know that #extend method can do this:

module X; def mm; puts 'extender' end end
Y = Class.new.extend X
X.mm # works

But I am writing a mixin (or, rather, would like to write) containing both instance methods and class methods:

module Common
  def self.class_method; puts "class method here" end
  def instance_method; puts "instance method here" end
end

Now what I would like to do is this:

class A; include Common
  # custom part for A
end
class B; include Common
  # custom part for B
end

I want A, B inherit both instance and class methods from Common module. But, of course, that does not work. So, isn’t there a secret way of making this inheritance work from a single module?

It seems inelegant to me to split this into two different modules, one to include, the other to extend. Another possible solution would be to use a class Common instead of a module. But this is just a workaround. (What if there are two sets of common functionalities Common1 and Common2 and we really need to have mixins?) Is there any deep reason why class method inheritance does not work from mixins?

Answer 1

A common idiom is to use included hook and inject class methods from there.

module Foo
  def self.included base
    base.send :include, InstanceMethods
    base.extend ClassMethods
  end

  module InstanceMethods
    def bar1
      'bar1'
    end
  end

  module ClassMethods
    def bar2
      'bar2'
    end
  end
end

class Test
  include Foo
end

Test.new.bar1 # => "bar1"
Test.bar2 # => "bar2"