Example:

  ### Controller
  class PostsController < ApplicationController
    autocomplete_for :user, :name
  end

    # Options
    By default, autocomplete_for limits the results to 10 entries,
    and sorts by the given field.
    
    autocomplete_for takes a third parameter, an options hash to
    the find method used to search for the records:
    
      autocomplete_for :user, :name, :limit => 15, :order => 'created_at DESC'
      
    # Block    
    with a block you can generate any output you need(passed into render :inline):
      autocomplete_for :post, :title do |items| 
        items.map{|item| "#{item.title} -- #{item.id}"}.join("\n")
      end
      
  ### View
  <%= f.text_field :auto_user_name, :class => 'autocomplete', 'autocomplete_url'=>autocomplete_for_user_name_users_path 
  %>

  ### Routes
  map.resources :users, :collection => { :autocomplete_for_user_name => :get}
  
  ### Records
  class User
    find_by_autocomplete 'name' --> self.find_by_autocomplete_name
  end
  class Post
    has_one :user
    autocomplete_for('user','name') --> auto_user_name= + auto_user_name
  end
  
  add autocomplete_for to your models to directly transform field values to objects
  and transform these objects back into field values for display
  - uses find_by_autocomplete_#{name} to find the wanted record
  - unfound record -> nil
  - blank string -> nil
    
  ### JS
  use any library you like 
  (includes examples for jquery jquery.js + jquery.autocomplete.js + jquery.autocomplete.css )
  jQuery(function($){//on document ready
    //autocomplete
    $('input.autocomplete').each(function(){
      var input = $(this);
      input.autocomplete(input.attr('autocomplete_url'));
    });
  });

Copyright (c) 2008 Michael Grosser, released under the MIT license
   
Original: Copyright (c) 2007 David Heinemeier Hansson, released under the MIT license