ruby on rails named scopes (searching)

I have a named scope (full_name) combination of first and last name and I'm wanting to use this in a search box.

I have the code below:

named_scope :full_name, lambda { |fn| {:joins => :actor, :conditions => ['first_name LIKE ? OR second_name LIKE ?', "%#{fn}%", "%#{fn}%"]} }

def self.search(search)
  if search
    self.find(:all, :conditions => [ 'full_name LIKE ?', "%#{search}%"])
  else
    find(:all)
  end
end

but this doesn't work as it gives the following error:

SQLite3::SQLException: no such column: full_name: SELECT * FROM "actors" WHERE (full_name LIKE '%eli dooley%')

Thanks in advance

Houlahan