Can you have a relationship that doesn't point to anything.

That is I have a tree structure of Objects and relationships to parents
and children.

But at the root node, there will be no parent which means the relationship will not point to anything. And if I try to access this relationship traversing back up the tree is there going to be some
strange exception?

And and the base of the tree there will also be NO relationship downward.

Is there any special way to do this.

Also is there a way to specify in the ejb-jar.xml that the
relationship is REQUIRED to create the object?