Marshall errors for association collection callbacks

Using the Proc based callbacks eg. has_many :somethings, :after_add => Proc.new{|r,something| something.do! } raises a marshall dump error ( procs can't be marshalled ) when a association collection is cached with the record.

Rewriting the callbacks to has_many :somethings, :after_add => :handle_adding_something bypasses this problem.

Perhaps it would be useful to catch this marshall error and raise a more informative exception.

Thougths?