Consulta MongoDB para todos os documentos com campo exclusivo

> db.foo.insert([{age: 21, name: 'bob'}, {age: 21, name: 'sally'}, {age: 30, name: 'Jim'}])
> db.foo.count()
3
> db.foo.aggregate({ $group: { _id: '$age', name: { $max: '$name' } } }).result
[
    {
        "_id" : 30,
        "name" : "Jim"
    },
    {
        "_id" : 21,
        "name" : "sally"
    }
]