Why does the conditional care about the execution code despite not returning true?

Question

I have this code conditionally displaying the project's title if we're in a view that has the @project instance variable:

<title>My app <%= "- " + @project.title if @project.present? %></title>

But it gives me an error with the following message: "can't convert nil into String", i.e. it tries to execute my string concatenation despite the conditional statement being false.

Why does it even bother?

Answer 1

You check if a project is present

@project.present?

but you don't check if the value of its title property is set. When title is nil, the following concatenation

"- " + @project.title

will fail with the can't convert nil into String error because you are effectively trying to concatenate

"- " + nil

that are two different types. You must deal with this case explicitly or use nil-to-string casting by using the interpolation

<%= "- #{@project.title}" if @project.present? %>

Moreover, beware of precedences. Embedding if conditions in that way with a print, may lead to obscure bugs caused by evaluation precedence.