Rails ignore "nil" values in loop

Question

I'm still pretty new to rails and this seems pretty basic but I can't find a straight answer.

I have 4 models:

def Order
  has_many :payments

def Payment
  belongs_to :order
  has_one :collection
  has_one :dispute

def Collection
  belongs_to :payment

def Dispute
  belongs_to :payment

The "show" Order page has the order details and a loop for all payments:

<% @order.payments.each do |f| %>
<div class="well">
  <p> Payment Date: <%= f.date_created %> </p>
  <p> Amount: <%= f.amount %> </p>
    ....# some other fields
  <p> Collection Date: <%= f.collection.date_created %> </p>
  <p> Disputed Date: <%= f.dispute.date_created %> </p>
</div>

This all works perfect unless there is any value missing in this loop. Which is a lot since many payments don't have "collections" or "disputes". When any value is missing I get the following error:

NoMethodError in Orders#show
undefined method `whatever value is missing' for nil:NilClass

Ideally the missing field would simply render blank, but I'm not sure why it breaks the whole view. Any help would be greatly appreciated!

vee · Accepted Answer · 2013-12-06 21:13:53Z

2

Have a look at Object#try method, and here is how you'd use it:

<% @order.payments.each do |f| %>
<div class="well">
  <p> Payment Date: <%= f.try(:date_created) %> </p>
  <p> Amount: <%= f.try(:amount) %> </p>
    ....# some other fields
  <p> Collection Date: <%= f.try(:collection).try(:date_created) %> </p>
  <p> Disputed Date: <%= f.try(:dispute).try(:date_created) %> </p>
</div>

Using try like above will make sure that you are not thrown an error, instead it renders blank as you want.

Update:

As @muistooshort has commented, try should be used cautiously as potential bugs could get swallowed unknowingly. Use try only where it's required; in this case f.try(:dispute) and f.try(:collection).

edited Dec 6, 2013 at 21:13

answered Dec 6, 2013 at 20:53

vee

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5 Comments

mu is too short Over a year ago

Putting try on them all looks like a good way to hide bugs, if f.nil? then @order.payments is buggy and should be fixed. Narrowly applying try just on the f.collection.date_created and f.dispute.date_created calls would make more sense, no?

Dogweather Over a year ago

I believe the .try should go on the object you suspect could be nil. So I'd write, e.g., f.collection.try(:date_created). If the object f itself might be nil, and thus we don't want to show anything, I'd use this at the top of the loop: next if f.nil?

vee Over a year ago

@muistooshort, yes selectively applying try on attributes would be the way to go. @user2815613, please have a look at @muistooshort's comment.

user2815613 Over a year ago

This worked, thanks! I do agree with that something might be buggy, so I'll try to play around with "try" on the other fields and see if I can isolate the issue.

mu is too short Over a year ago

I was thinking more along the lines of f.collection.try(:date_created) and f.dispute.try(:date_created).

Edward · Accepted Answer · 2013-12-06 20:52:59Z

0

You can use the try method -http://api.rubyonrails.org/classes/Object.html#method-i-try

For example, replace

<%= f.date_created %>

with

<%= f.try(:date_created) %>

answered Dec 6, 2013 at 20:52

Edward

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Comments

Leszek Andrukanis · Accepted Answer · 2013-12-06 21:05:01Z

0

You can also use NullObject pattern. It's more complex but handling blank value in every possible case can leads you to a lot of spaghetti code. Moreover from my experience try method is often overused. Take a look: http://devblog.avdi.org/2011/05/30/null-objects-and-falsiness/

answered Dec 6, 2013 at 21:05

Leszek Andrukanis

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