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'

For help on defining text input fields with autocompletion, see ActionView::Helpers::JavaScriptHelper.

For more examples, see script.aculo.us:

Methods
Public Instance methods
auto_complete_for(object, method, options = {})
    # File vendor/rails/actionpack/lib/action_controller/macros/auto_complete.rb, line 37
37:         def auto_complete_for(object, method, options = {})
38:           define_method("auto_complete_for_#{object}_#{method}") do
39:             find_options = { 
40:               :conditions => [ "LOWER(#{method}) LIKE ?", '%' + params[object][method].downcase + '%' ], 
41:               :order => "#{method} ASC",
42:               :limit => 10 }.merge!(options)
43:             
44:             @items = object.to_s.camelize.constantize.find(:all, find_options)
45: 
46:             render :inline => "<%= auto_complete_result @items, '#{method}' %>"
47:           end
48:         end