mixin.rb

module Foo
  def self.included(base)
    base.extend(ClassMethods)

    # alternative
    def base.class_name
      "class method name"
    end
  end

  def instance_name
    "instance method name"
  end

  module ClassMethods
    def find
      "findddddd"
    end
  end
end

class Person
  include Foo
end

p = Person.new
p.instance_name # => "instance method name" 

Person.class_name # => "class method name"
Person.find # => findddddd