How to Handle Nil Errors in Rails?

Question

What is the correct way to deal with nil errors in Rails? I often get errors like:

NoMethodError (undefined method `questions' for nil:NilClass):

For example, let's say I have chapters in my app which each have a question. I would like to call the question from the previous chapter from within the current question, so I write the following code in question.rb:

def previous_question
 self.chapter.previous.question
end

This might cause the above-mentioned error, so I write a method in the model to check if this will lead to a nil result:

def has_previous_question?
 self.chapter and self.chapter.previous and self.chapter.previous.question
end

If I make sure to call that before calling previous_question , it can work, but it looks ridiculous. Is there a better way to deal with nil errors in Rails?

Answer 1

I could not tell this is THE right way but it is another way of handling this kind of situation:

def previous_question
  self.chapter.previous.try(:question)
end

That way there won't be any error, if there is no previous chapter, the method will simply return nil .

If you want to return something else in case it is actually nil, you can write:

def previous_question
  self.chapter.previous.try(:question) || returning_this_value_instead
end

Side note: you don't need to use self in that kind of situation:

def previous_question
  chapter.previous.try(:question) || returning_this_value_instead
end

Answer 2

One of my favorite view rendering methods:

http://apidock.com/rails/Object/try

Answer 3

有趣的方法是使用nil对象 ，尽管它们并不适合所有情况。