Using the null object pattern

This is a common problem, you have an object, and that object does some logic, and then you find that at some point this logic needs a special condition, because the object is null, or somet variable inside it that holds another variable is null, here’s a small example Then in the view we do something like this

But then we find that sometimes this book doesn’t exist for some reason, so we add this condition

This works, but it’s considered a bad smell and an antipattern, first of all you added the logic of handling non existent books in the view, and if we have multiple partials each will have this block, a smiple change would require going through all these templates.

Instead we could use what’s called a null object , which is an object from a different class but has the same interface of the original class, and would respond to the same method, and returns the case specific response that we want.

For our case a simple way to impelemt this is by returning a NullBook instance for Orders that don’t have a book, when we try to read the book’s title, the NullBook will respond with No books exist, so the view will handle it normally thinking that it’s the actual book.

First we just create a NullBook class and place this logic inside it

Then in the Order class we will modify the getter a little bit

This way if @book is null then we return an instance of NullBook, now we can clean up the view back to this