I'm following the Kevin Skoglund tutorial Ruby on Rails 3 Essential Training, which was written for rails 3.0, though I am currently using 3.2. It uses the following method in the pages_controller with a before_filter to display only the pages which belong to the parent subject.

The tutorial explicitly uses .find_by_id because if the result is nil it "will not return an error". However I get the "Called id for nil, which would mistakenly be 4 -- if you really wanted the id of nil, use object_id" error when trying to view a page where @subject has been set to nil.

def find_subject
  if params[:subject_id]
    @subject = Subject.find_by_id(params[:subject_id])
  end
end

The actual code that is causing the error is:

def list
  @pages = Page.order("pages.position ASC").where(:subject_id => @subject.id)
end

Is this something that has changed since 3.0? If so, what would be the correct way to implement this functionality in 3.2?