How to sort nested object - mongodb

Question

db.sort.drop();
db.sort.insert({stats: [{userId: 1, date: '01012013'},{userId: 2, date: '31122012'}]});
db.sort.insert({stats: [{userId: 1, date: '31122013'},{userId: 2, date: '01012012'}]});
> db.sort.find({'stats.userId': 1}).sort({'stats.date': 1}).pretty()
{
"_id" : ObjectId("52af1ce974be7dbd071e8563"),
"stats" : [
{
"userId" : 1,
"date" : "31122013"
},
{
"userId" : 2,
"date" : "01012012"
}
]
}
{
"_id" : ObjectId("52af1ce974be7dbd071e8562"),
"stats" : [
{
"userId" : 1,
"date" : "01012013"
},
{
"userId" : 2,
"date" : "31122012"
}
]
}
How to get the documents sorted by date userId: 1?
I expect to see:
{
"_id" : ObjectId("52af1ce974be7dbd071e8562"),
"stats" : [
{
"userId" : 1,
"date" : "01012013"
},
{
"userId" : 2,
"date" : "31122012"
}
]
}
{
"_id" : ObjectId("52af1ce974be7dbd071e8563"),
"stats" : [
{
"userId" : 1,
"date" : "31122013"
},
{
"userId" : 2,
"date" : "01012012"
}
]
}

Answer 1

Something along the lines of:
db.COLLECTION.find({stats.userID: 1}).sort({'stats.date':1})
Given your data above, when I execute that query, I get this back:
"stats" : [
{
"userId" : 1,
"date" : 1012013
}

Answer 2

You could use the aggregation framework to achieve
db.sort.aggregate(
[
{$unwind:"$stats"},
{$match:{"stats.userId":1}},
{$sort:{"stats.date":1}}
]);
If you could change your schema to store the user+id combination as the field name with the date as its value then you accomplish this easily
> db.so2.find().pretty()
{
"_id" : 1,
"stats" : [
{
"userId1" : "01012013"
},
{
"userId2" : "31122012"
}
]
}
{
"_id" : 2,
"stats" : [
{
"userId1" : "31122013"
},
{
"userId2" : "01012012"
}
]
}
You then change the query predicate from userId : 1 to check for existence of the userId field
> db.so2.find({"stats.userId1":{$exists:1}}).sort({"stats.userId1":1}).pretty()
{
"_id" : 1,
"stats" : [
{
"userId1" : "01012013"
},
{
"userId2" : "31122012"
}
]
}
{
"_id" : 2,
"stats" : [
{
"userId1" : "31122013"
},
{
"userId2" : "01012012"
}
]
}