From OR to CR modeling - Hierarchies

So in the previous post, I started to think about data models in a Content Repository and the differences between relational models and, well, what?

But first a disclaimer: I am no big content repository expert. So expect me to do some classic mistakes, and ask some rather naive questions.

Here be one:

Is a hierarchy imperative to content modeling with JCR?

My thought is that hierarchical modeling feels natural in JCR. The hierarchical structure is instrinsic to structures with nodes, parents and children. So the next question would be

How do you map the hierarchies of an existing relational model to a hierarchical one?

We have the classical example once again

class Author {
 static hasMany = [ books : Book ]
 String name
}
class Book {
 static belongsTo = [author:Author]
 String title
}

which maps naturally to

/authors
/stephen king
     /books
         /Misery

But what if there is another hierarchy, e.g.

class Author {
 static hasMany = [ books : Book ]
 String name
}
class Book {
 static belongsTo = [author:Author, category: Category]
 String title
}

class Category {
 static hasMany = [ books : Book ]
}

Now the books suddenly are part of 2 hierarchies. How to model this? I currently see 2 possibilities

1. Have one leading hierarchy
2. Flat it out

/authors
 /stephen king
      /books
          /Misery
/categories
 /Thriller
      /Misery*

vs

/authors
 /stephen king
      /books
          /Misery*
/categories
 /Thriller
      /books
          /Misery*
/books
 /Misery

(the * depicts a relation to a Node elsewhere)

Another solution would be to allow multiple dimensions, in which the objects can be stored (this is more or less the same as the "flat out" version, except that looking at the object always gives you the "real" one, and not the relation.

As I said, I am not an expert of data modeling.

Feel free to comment on it...

Ok, next post will be about Groovy/Grails again :) (yeah, with a dash of JCR in it)