Question

module A; def a; end; end
module B; def b; end; end

class C; include A; end

module A; include B; end
class D; include A; end

C.new.b # undefined method error
D.new.b # nil

C.ancestors # [C, A, Object...]
D.ancestors # [D, A, B, Object...]

How to include module B inside of A, so that classes that already include module A will also get methods from module B?

Answer 1

You can’t.

When you include a mixin M into a class C, Ruby creates a new class ⟦M′⟧ whose method table points to the method table of the mixin M and whose superclass is the superclass of C, then makes this class the new superclass of C. This is repeated for every mixin that was mixed into M.

Note that this algorithm is run only once, when you mix M into C. Modules that get included later, will not get considered.