Can't find using instance method as field

Hi, I just started experimenting with acts_as_ferret. It’s working great
if I’m searching through database fields through a setup like:

acts_as_ferret :fields => [‘title’]
Page.find_by_contents(‘Blue’)

But if I try searching through an instance method, I get no results
back. Anybody know why?

acts_as_ferret :fields => [‘extra’]
def extra
‘Example’
end

after deleting the index directory, running “reload!” in the console:

Page.find_by_contents(‘Example’)
=> #<ActsAsFerret::SearchResults:0x2883ba8 @results=[], @total_hits=0>

It is reindexing since the development.log has many lines like:
creating doc for class: Page, id: 9
Adding field extra with value ‘Example’ to index
creating doc for class: Page, id: 10
Adding field extra with value ‘Example’ to index

It seems that I needed to exit and reenter the irb console before
changes are noticed.