Rails3 custom data from controller

I have some controller (tasks) and wanted to collect requester mail
(with formtastic) of this task (some user). Task model has
requested_by_id field. Now, I made a view for task edit with all
fields, but I don't know how to show requestere's email (association) so
I can use autocomplete. I'm breaking my head with that for 2 days

So, I want to show text box so I can enter users email (later will be
autocomplete), find is't ID end save it with new/edited taks.

Thanks in advance