CODEWITHSUNDEEP
ruby-on-railsrubyexception
po.studio
po.studio

Reputation: 4127

Exception Handling: "undefined method `____' for nil:NilClass"

One of the most common reasons my web application fails is because a user sometimes lacks a certain attribute that a view expects it to have. For instance, most users in my application have an education (school, degree, etc.) entry in our system, but some users do not. Assuming my view looks something like this:

<% @educations.each do |education| %>
  <%= education.school %>
  <%= education.degree %>
<% end %>

I want to avoid "Pokemon" exception handling and feel that there has to be a better way around dealing with a "undefined method `degree' for nil:NilClass" error in the case that a user does not have an education entry in our database. This just seems like an ugly/tedious fix:

<% @educations.each do |education| %>
  <% if education.school %>
    <%= education.school %>
  <% end %>
  <% if education.degree %>
   <%= education.degree %>
  <% end %>
<% end %>

Any input is appreciated. Thank you!

Upvotes: 2

Views: 1349

Answers (2)

Brad Werth
Brad Werth

Reputation: 17647

You have a couple of options here.

The easiest to implement is the try method. It is used like so:

<%= education.try( :degree ) %>

The problem is that try() is viewed as a bit of an anti-pattern. As the reference indicates, you can achieve similar functionality with something like:

<%= education && education.degree %>

This isn't really a lot different, intellectually, in my opinion. A popular way of handling this a little more cleanly is the Null Object pattern, which is basically an object with defined neutral ("null") behavior.

Upvotes: 0

Andrew
Andrew

Reputation: 43153

As long as you know the first object you're working on won't be nil, the easiest way is to just do this:

- @educations.each do |education|
  = education.try :school
  = education.try :degree

The #try method is pretty handy. You can also call .to_s on anything you think might be nil, Ie:

- @educations.each do |education|
  = education.school.to_s
  = education.degree.to_s

This will convert nils to an empty string. This isn't as useful in the view IMO, but comes in handy a lot of times if you have input that is expecting to be a string and might be empty. Ie a method like:

def put_in_parenthesis(string)
  "(" + string.to_s + ")"
end

Upvotes: 1

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