An example of using nested docs. Or basically a hash table instead of an array of js objects.

nested_docs.js

var x = {_id: 1, 'tags' : { 'castle' : { 'n' : 10 }, 'age' : { 'n' : 2 } } };
var y = {_id: 2, 'tags' : { 'keep' : { 'n' : 2 }, 'castle' : { 'n' : 6 } } };
var z = {_id: 3, 'tags' : { 'keep' : { 'n' : 5 }, 'cats' : { 'n' : 6 } } };
db.doc.save(x);
db.doc.save(y);
db.doc.save(z);

// find all docs
db.doc.find();

// find only docs tagged castle
db.doc.find({'tags.castle' : {$exists:true}});

// find docs tagged castle, sort by value of 'n' in castle
db.doc.find({'tags.castle' : {$exists:true}}).sort({'tags.castle.n':-1});

// as above, but return less fields
db.doc.find({'tags.castle' : {$exists:true}}, {_id:1, 'tags.castle':1}).sort({'tags.castle.n':-1});