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I wanted to show a link in a Rails application based on whether a person was current_user.admin? and signed_in?

The code looked something like this:

= link_to "Admin Menu", root_path if current_user.admin? && signed_in?

That would work when I was signed in but as soon as I signed out, I’d get a no method error.

That happens because when you’re signed out, there is no current_user.

To fix this issue, I wrote a more verbose statement:

- if signed_in?
  - if current_user.admin?
    = link_to "Admin Menu", root_path</pre>

I first checked whether a user was signed_in?. If they were, then the current_user would be available and I could use it.

It works but we can shorten this code by understanding how the ruby && operand is evaluated.

The && operand goes through conditions in order. As soon as it receives a false, it returns false.

I had problems when evaluating the line below because current_user didn’t exist and Ruby raised an error.

# No method error
current_user.admin? && signed_in?

However, when I switched the order of these two statements, the error went away.

# works perfectly
signed_in? && current_user.admin?

This works because the method signed_in? will always return a true or false, regardless of whether we’re signed in or not.

When signed_in? is true, Ruby continues to evaluate the second statement current_user.admin?.

When signed_in? is false, Ruby doesn’t even look at current_user.admin? and simply returns false. This is why the line below works:

= link_to "Admin Menu", root_path if signed_in? && current_user.admin?

Safe navigation operator and try

Another alternative is to use the safe navigation operator. If current_user doesn’t exist, you can ask Ruby to fail silently with a nil instead of a no method error.

You can call the try method and pass in the method you’re checking. If it doesn’t exist, Ruby will return nil instead of an error.

In Ruby 2.3, you can use the shorthand safe navigation operator &. Behind the scenes, & invokes the try! method.

current_user.blah? # No Method Error
current_user.try(:blah) #nil
current_user&.blah #nil

You can also daisy chain try and &

current_user.try(:blah).try(:fake_method) #nil
# or use the safe navigation operator
current_user&.blah&.fake_method #nil