definitions.rb

class MyClass
  def foo
    puts "MyClass -> foo"
    bar
  end
  
  def bar
    puts "MyClass -> bar"
  end
end

class YourClass < MyClass
  def bar
    puts "YourClass -> bar"
  end
end

class MyWrapper < Struct.new(:obj)
  def foo
    @obj.foo
  end
  
  def bar
    puts "MyWrapper -> bar"
  end
end

usage.rb

myClass = MyClass.new
myClass.bar
#=> "MyClass -> bar"

yourClass = YourClass.new
yourClass.bar
#=> "YourClass -> bar"

myWrap = MyWrapper.new(myClass)
myWrap.bar
#=> "MyWrapper -> bar"

# So far, so good.

yourClass.foo
#=> "MyClass -> foo"
#=> "YourClass -> bar"

# So it calls the original foo, and then my overridden bar.  Good.

myWrap.foo
#=> "MyClass -> foo"
#=> "MyClass -> bar"

# But we're trying to replace the original bar, but anything that calls into it hits
# the original, not our wrapped version.