This is a pragmatic piece of Fowler on the rather dry topic of Object-relational mappings - in short, the attempt to marry an object-oriented code base with a relational data base.
Usually you’d get enough early success to commit deeply to the framework and only after a while did you realize you were in a quagmire - this is where I sympathize greatly with Ted Neward’s famous quote that object-relational mapping is the Vietnam of Computer Science
What Fowler refers to here, is Ted Neward’s article “The Vietnam Of Computer Science”
I don’t see why you couldn’t have relations in a object model database. Just allow fields that are references to other objects or their keys.