Create new properties based on embedded array elements in MongoDB - mongodb

Question

I have this kind of document
{
"_id" : ObjectId("573342930348ce88ff1685f3"),
"presences" : [
{
"_id" : ObjectId("573342930348ce88ff1685f2"),
"createdAt" : NumberLong(1458751869000)
},
{
"_id" : ObjectId("573342930348ce88ff1685f5"),
"createdAt" : NumberLong(1458751885000)
},
{
"_id" : ObjectId("573342930348ce88ff1685f7"),
"createdAt" : NumberLong(1458751894000)
}
]
}
How can I extract first and last presences element to new properties firstPresence and lastPresence like this?
{
"_id" : ObjectId("573342930348ce88ff1685f3"),
"firstPresence": {
"_id" : ObjectId("573342930348ce88ff1685f2"),
"createdAt" : NumberLong(1458751869000)
},
"lastPresence": {
"_id" : ObjectId("573342930348ce88ff1685f7"),
"createdAt" : NumberLong(1458751894000)
},
"presences" : [
...
]
}
I want to use a query that can be applied to all documents in one time.

Answer 1

You need to $unwind your presences array to do the aggregation. Before grouping you can sort them by createdAt to utilize $first and $last operators.
db.collection.aggregate(
[
{ $unwind: "$presences" },
{ $sort: { "presences.createdAt": 1 } },
{
$group: {
_id: "$_id",
"presences": { $push: "$presences" },
"lastPresence": { $last: "$presences" },
"firstPresence": { $first: "$presences" },
}
},
{ $out : "collection" }
])
Last aggregation pipeline ($out) will replace existing collection.

Answer 2

According to above mentioned description as a solution to it please try executing following aggregate query into MongoDB shell
db.collection.aggregate(
// Pipeline
[
// Stage 1
{
$project: {
first: {
$arrayElemAt: ["$presences", 0]
},
last: {
$arrayElemAt: ["$presences", -1]
},
presences: 1
}
},
]
);