Example:

  # Controller
  class BlogController < ApplicationController
    auto_complete_for :post, :title
  end

  # View
  <%= text_field_with_auto_complete :post, title %>

By default, auto_complete_for limits the results to 10 entries,
and sorts by the given field.

auto_complete_for takes a third parameter, an options hash to
the find method used to search for the records:

  auto_complete_for :post, :title, :limit => 15, :order => 'created_at DESC'
  
Included in the plugin is the text_field_with_auto_complete helper macro.
This makes using this plugin a snap. 

In your controller, you'll need to define an action called
auto_complete_for to respond the AJAX calls, or pass in the
tag option :controller => "some_controller" if you are calling
a different controller's auto_complete_for action.

An example of why you would want to pass in the controller is if
you have a search controller that is called from many places in 
your site. As long as you have a method such as auto_complete_for_search_query
in your search controller, you can call this method from anywhere like this:

text_field_with_auto_complete :search, :query, :controller => "search" 

If you do not pass in the :controller option, the current controller is assumed.
For instance, if you happened to be on a page served by the search controller, 
you can simply call the following, given our previous example:

text_field_with_auto_complete :search, :query

There are many other useful methods, so be sure to check out the comments in the
source.

For more examples, see script.aculo.us:
* http://script.aculo.us/demos/ajax/autocompleter
* http://script.aculo.us/demos/ajax/autocompleter_customized

Copyright (c) 2007 David Heinemeier Hansson, released under the MIT license 

Additional functionality and comments Copyright (c) 2009, Chris Beck  
Released under the MIT license