gistfile1.rb

def search
  @artists = Artist.find_all_by_festival_id(@festival.id, :conditions => ["name LIKE ?", "%#{params[:q]}%"])
  respond_to do |wants|
    wants.js { render :json => @artists }
  end
end

gistfile2.js

$("#artist_picker").autocomplete("/artists/search", {
	formatItem: function(item){
		return item.artist.name;
	}
}).result(function(event, item) {
	alert("The id is: " + item.artist.id)
});