auto_complete_association_jquyer.rb

module AutoCompleteJquery      
  
  def self.included(base)
    base.extend(ClassMethods)
  end

  module ClassMethods

    def auto_complete_association_for(object, klass, attribute, options = {})
      define_method("auto_complete_for_#{object}_#{klass}_#{attribute}") do
        klass_constant = klass.to_s.camelize.constantize

        @items = klass_constant.connection.select_values(%Q[
                                                            SELECT #{attribute}
                                                            FROM #{klass_constant.table_name}
                                                            WHERE #{attribute}
                                                            LIKE '%#{params[:q].downcase}%'
                                                            LIMIT 10
                                                           ])

        render :text => @items.join("\n")
      end
    end

  end
  
end

items_controller.rb

class ItemsController < ApplicationController
  auto_complete_association_for :item, :category, :name
end