issues with searching custom fields with acts_as_ferret

Hey all,

I'm using acts_as_ferret this way:
class Job < ActiveRecord::Base
acts_as_ferret :fields => [:title, :workers_name]

def workers_name
return self.workers.inject("") {|names,b| names + " " + b.first_name}.to_s

But when I do Job.find_by_contents("workers_name:patrick")
I get nil.

Yet when I do:
j=Job.find :first
I do get a worker (among others) which name is patrick.

Any idea why I don't get it with find_by_contents?

thanx in advance