search for: attachment_observer

...s. In rails you define your observers in the environment.rb and it instantiates them somewhere during startup. To use that in merb you have to define the observers and then also instantiate them. Add something like this to the end of the merb_init.rb file. ActiveRecord::Base.observers << :attachment_observer ActiveRecord::Base.instantiate_observers At least its working for me from merb -i right now.