Question

I have a very concrete dilemma right now.

Given the following models:

class Message < ActiveRecord::Base
  attr_accessible :body, :sent_at

  belongs_to :subject
end

class Subject < ActiveRecord::Base
  attr_accessible :title

  has_many :messages
  belongs_to :last_message, :class_name => 'Message', :foreign_key => 'last_message_id'
end

In a view I want to iterate over a list of subjects and display:
– Subject title
– sent_at for the subject’s last message
like this:

<% @subjects.each do |subject| %>
  <%= subject.title %>
  <%= subject.last_message.sent_at %>
<% end %>

The thing is: subject.last_message may some times be nil. In which case, the above code will throw an exception.

So: What is the best solution to this? I can see 3 possibilities, but honestly don’t know which are considered good or bad.

1) Let the view rescue it

<%= subject.last_message.sent_at rescue '' %>

2) Make a helper

def last_message_sent_at(subject)
  return '' if subject.last_message.blank?
  subject.last_message.sent_at
end

<%= last_message_sent_at(subject) %>

3) Make a sort of “proxy” on the Subject model

class Subject < ...
  ...
  def last_message_sent_at
    return '' if last_message.blank?
    last_message.sent_at
  end
end

<%= subject.last_message_sent_at %>

Which would you choose, and why? Or is there perhaps another way, which I haven’t thought about?

/ Carsten

Answer 1

Use try :

<%= subject.last_message.try(:sent_at) %>

So, if subject.last_message is nil, you will get no output; else if it is not nil, it will call the method sent_at on subject.last_message.

It is like a convenient form for your #2 idea

Documentation

As additional thought, helper is a bad choice. You ideally always want a “receiver” (in some_class.perform(), some_class is the “receiver” i.e. it “receives” the message “perform”). I avoid Helpers unless I need to generate HTML. So, your #3 does have a receiver, but since Rails provides try, you do not need to roll your own.