Check whether an attribute of a model exists

Question

In Rails, I am getting an error when I try to evaluate whether the user has entered a brand for their computer:

if @user.computer.brand.empty?

NoMethodError (undefined method 'brand' for nil:NilClass):

If the user has not entered a computer, this will return an error that there is no method brand on nil class. What is the correct way to check that a user has both entered a computer and a brand for that computer?

simplest solution would be to just use `if @user.computer && @user.computer.brand.empty?` but a cleaner solution would be to use Null objects as specified in this article http://robots.thoughtbot.com/rails-refactoring-example-introduce-null-object — jvnill, Apr 02 '14 at 05:22

score 2 · Accepted Answer · answered Apr 02 '14 at 05:25

2

Try with try:

@user.computer.try(:brand)

This will return nil if either computer or computer.brand is nil, or it will return the assigned brand.

answered Apr 02 '14 at 05:25

vee

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score 0 · Answer 2 · answered Apr 02 '14 at 05:20

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if @user.computer && @user.computer.brand.present?

answered Apr 02 '14 at 05:20

Matteo Melani

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Add some explanation to answer please so other as well as user can understand what it does. – InnocentKiller Apr 02 '14 at 05:55

score 0 · Answer 3 · edited May 23 '17 at 12:11

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if @user.computer.present? && @user.computer.brand.present?

You can also use ! nil? instead of present.

See this post - very useful

edited May 23 '17 at 12:11

Community

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answered Apr 02 '14 at 05:23

Marco Prins

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Check whether an attribute of a model exists

3 Answers3