I understand that you can use MongoDB in a relational fashion, but I don't get how this is showing why. The mongo docs have examples showing what this blog says can't be done.

http://www.mongodb.org/display/DOCS/Retrieving+a+Subset+of+F...

> t.find({}, {'x.y':1}) { "_id" : ObjectId("4c23f0486dad1c3 a68457d20"), "x" : { "y" : 1 } }

And the mongo docs even have an example pretty detailing what this page wants (paging of comments)

db.posts.find({}, {comments:{$slice: -5}}) // last 5 comments

So, what am I missing?