callbacks don't work from extended modules

(from [be078ee162fcae883a5621a30929879cd783a238]) Run callbacks from object's metaclass [#575 state:resolved]

http://github.com/rails/rails/co...

We had to revert this. Apparently it broke a ton of shit.

Maybe we don't want callbacks on the metaclass, but a set of callbacks for objects as well as classes.

Lets try a new API

module ModuleWithCallbacks
  def self.extended(obj)
    obj.before_save :module_callback
  end

  def module_callback
    # ...
  end
end

p = Person.new
p.extend(ModuleWithCallbacks)

# or also do
p.before_save :module_callback
def p.module_callback
  ...
end

We could have a instance variable on the object that tracks its callbacks

It looks like this is going take alot more work. If you ever decide its worth pursuing, open and new ticket for refactoring Callbacks and assign it to me.