Node reference and Views 2: How to define a "refers to nothing" filter

This was a problem I had some time ago, and I'm pretty sure this post will save time to some people.

The problem: Using a Node reference field and Views 2, how do I define a "refers to nothing" condition in the filters?

Solution by merlinofchaos: Add the node reference as a relationship. Then add the Node: Nid filter, set it to the relationship, and set the operator to Is Empty.

This method, changing the operator, can be used for the opposite: refers to any node.

It sounded so easy when merlinofchaos explained it to me...