The HeartbeatFor plugin will update a 'last_activity' datetime column for
one or more models.  It attempts to do this upon each request, but no more frequently
than the given interval to prevent unnecessary load on the database.
  
You only need specify a method that returns a model (with a 'last_activity' column by default)

  heartbeat_for :model_method [, :column [, :interval]]
    :column - a datetime column, defaults to "last_activity"
    :interval - minimum number of seconds between updates, defaults to 2.minutes

Place the call to heartbeat_for in a controller, e.g. application.rb

The default usage:
==================
  class ApplicationController < ActionController::Base
    # update the :last_activity column of :current_user at most every 2 minutes:
    heartbeat_for :current_user
  
    def current_user
      # return an instance of User
    end
  end

Specifying the column and interval:
======================================
  class ApplicationController < ActionController::Base
    # update the :some_datetime column of :my_model at most every 42 seconds:
    heartbeat_for :get_my_model, :some_datetime, 42.seconds
  end
  
You can also call heartbeat_for multiple times to update many models:
=====================================================================
  class ApplicationController < ActionController::Base
    heartbeat_for :current_foo
    heartbeat_for :current_bar, :last_seen_at
    heartbeat_for :current_baz, :last_activity, 5.minutes
  end