app/controllers/autocomplete_controller.rb

#
# Autocompletion for models
#
class AutocompleteController < ActionController::Base

  MIN_LENGTH = 3

  def index
    term = params[:term].to_s.strip

    if term.length < MIN_LENGTH
      @result = []
    else
      autocomplete = Autocomplete.new

      # add scopes
      params[:scope].to_s.split(",").each do |scope|
        exclude = params[:exclude] || {}
        autocomplete.add_scope(scope, exclude[scope] )
      end

      # do the search
      @result = autocomplete.search(term).map{|r| {
        id:    r.id,
        type:  r.class.to_s,
        value: r.to_s
      }}
    end
    render :json => @result
  rescue Autocomplete::InvalidScope => error
    render :text => error.message, :status => :unprocessable_entity
  end

end