composition_example.rb

module Foo
  class Bar
  
    attr_accessor :qux_service, 
                  :param1,
                  :param2
                  
     #initialize the class and dependencies
    def initialize(foo_klass = Foo)
      @foo_klass = foo_klass

      #initialize dependencies
      @qux_service = Foo::QuxService.new
    end

    def call(bar_id, param1, param2)
      @param1 = param1
      @param2 = param2
      
      call_qux_service
    end
    
    private
    
    def call_qux_service
      qux_service.(param1, param2)
    end
  end
end




module Foo
  class QuxService
  
    attr_accessor :qux_klass
    
    def initialize(qux_klass = Qux)
      @qux_klass = qux_klass
    end
    
    def call(param1, param2)
      param1 + param2
    end
  end
end

inheritence_example.rb

module Foo
  class Bar
  
    attr_accessor :param1,
                  :param2
                  
     #initialize the class and dependencies
    def initialize(foo_klass = Foo)
      @foo_klass = foo_klass
    end

    def call(bar_id, param1, param2)
      @param1 = param1
      @param2 = param2
      
      call_qux_service
    end
    
    private
    
    def call_qux_service
      ::QuxService.do_a_thing(param1, param2)
    end
  end
end



module QuxService
  extend self

    def do_a_thing(param1, param2)
      param1 + param2
    end
  end
end