> var item = {
	name: 'Sword'
};

> db.items.save(item);
> db.items.find();
{ "_id" : ObjectId("4e837cf157a44a81540783c9"), "name" : "Sword" }
> var u = {
	name: 'John',
	email: 'john@j.com',
	instance: { $ref : db.items, $id : ObjectId("4e837cf157a44a81540783c9") }
};
> db.users.save(u);
Wed Sep 28 17:22:32 InternalError: too much recursion shell/collection.js:126