Question

Say I have a User model, a Task model that belongs_to :user, :has_one :event, has a completed boolean attribute, and an Event model that is created when a task is completed, and also belongs_to :event.

In the TaskObserver I’ve noticed that instead of

# app/controllers/task_observer.rb
class TaskObserver < ActiveRecord::Observer
def after_update(task)
  def after_update
    task.create_event(:user=>task.user) if task.completed?
  end
end

I could write

task.create_event(:user_id=>task.user.id)

or even

task.create_event(:user_id=>task.user_id)

While the first way seems the most correct, are there benefits to using either of the latter variations?

Answer 1

In this specific case I went with task.create_event(:user_id=>task.user_id). By running:

$ rails c
ruby-1.8.7-p299 > ActiveRecord::Base.logger = Logger.new(STDOUT)
ruby-1.8.7-p299 > Task.where("user_id IS NOT NULL).user.id
...
User Load (1.2ms)  SELECT `users`.* FROM `users` WHERE (`users`.`id` = 103) LIMIT 1
=> 103

you can see that Rails will actually load the User from the database. Even if it was cached, I don’t see why I should handle the actual objects if I’m just copying a reference.

So in general, I think it’s preferred to use objects when you’ve used them before, and IDs when you didn’t and don’t plan to.