http://twitter.com/zombidev/status/164484915059113984

composition_vs_inheritance.md

#include and #extend have same effect on instances as inheritance

Let's say we have this

module A; end
module B; end
class C; end
class D < C; end

Then we modify instance

d = D.new
D.send(:include, A)
d.extend(B)
d_meta = class << d; self; end

Lets see what we got (method dispatch lookup table)

d.class.ancestors
=> [D, A, C, Object, Kernel, BasicObject]

d_meta.ancestors
=> [B, D, A, C, Object, Kernel, BasicObject]

So it's all up to how and what structure you see

and to me, benefits of composition over inheritance is that it doesn't modify object dispatch hierarchy.

PS: pls comment here.