DEPRECATION WARNING: This method will become a separate plugin when Rails 2.0 ships.

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
A
Instance Public methods
auto_complete_for(object, method, options = {})

Source: show 

# File rails/actionpack/lib/action_controller/macros/auto_complete.rb, line 38
def auto_complete_for(object, method, options = {})
  define_method("auto_complete_for_#{object}_#{method}") do
    find_options = { 
      :conditions => [ "LOWER(#{method}) LIKE ?", '%' + params[object][method].downcase + '%' ], 
      :order => "#{method} ASC",
      :limit => 10 }.merge!(options)
    
    @items = object.to_s.camelize.constantize.find(:all, find_options)

    render :inline => "<%= auto_complete_result @items, '#{method}' %>"
  end
end