Example of conditionally mixing in Instance Methods if the base klass implements certain instance methods

conditional_mixin.rb

require 'rails/all'
require 'rspec'
require 'pry'

module MyMixin
  extend ActiveSupport::Concern

  included do
    raise NotImplementedError.new("#{self} must implement '#bar'.") unless self.instance_methods.include?(:bar)
    send(:include, MixinInstanceMethods)
  end

  module MixinInstanceMethods
    def hai
      "Hai Mike!"
    end
  end
end

class Foo
  attr_accessor :bar
  include MyMixin
end

f = Foo.new
f.respond_to?(:bar).should == true
f.hai.should == "Hai Mike!"

Commenting out the 'getter' for :bar will cause

conditional_mixin.rb:9:in `block in <module:MyMixin>': Foo must implement '#bar'. (NotImplementedError)