Embedded List ordering - mongodb

Question

I have the following object with an embedded list of items and I would like to write a query and return all or specific items ordered by date. Is it possible to do it or should I have a different collection for items and keep here their references?
I know that you can match specific element using $elemMatch.
{
"_id": "51cb12857124a215940cf2d4",
"level1":
[
{
"name":"item00",
"description":"item01",
"date": 1238492103
},
{
"name":"item10",
"description":"item11",
"date": 1238492104
}
]
}

Answer 1

If you want these items ordered by date on a more often than not then your best option is to keep the list ordered in the first place. The $push operator has an additional $sort paramter explicitly for this purpose.
db.collection.update(
{ "_id": "51cb12857124a215940cf2d4" },
{ "$push": {
"level1":{
"$each":[{
"name":"item11",
"description":"item11",
"date": 1238492104
}],
"$sort": { "date": 1 }
}
}
)
That actually even adapts so you could just sort your whole collection in one statement:
db.collection.update(
{},
{ "$push": {
"level1":{
"$each":[], "$sort": { "date": 1 }
}
},
{ "multi": true }
)
Without that your ony alternate is to order the results via the .aggregate() method. This really should not be your chosen operation as it requires processing $unwind on the array contents and then $sort operation on the elements within the document. Naturally this comes with some significant overhead on larger selections:
db.collection.aggregate([
{ "$unwind": "$level1" },
{ "$sort": { "_id": 1, "level1.date": 1 } },
{ "$group": {
"_id": "$_id",
"level1": { "$push": "$level1" }
}}
])