ruby script/plugin install http://svn.rubyonrails.org/rails/plugins/auto_complete/

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'

There is currently no advanced documentation for this plugin.