MongoDb aggregation framework value of a field where max another field - mongodb

Question

I have a collection that has records looking like this:
"_id" : ObjectId("550424ef2f44472856286d56"), "accountId" : "123",
"contactOperations" :
[
{ "contactId" : "1", "operation" : 1, "date" : 500 },
{ "contactId" : "1", "operation" : 2, "date" : 501 },
{ "contactId" : "2", "operation" : 1, "date" : 502 }
]
}
I want to know the latest operation number that has been applied on a certain contact.
I'm using the aggregation framework to first unwind the contactOperations and then grouping by accountId and contactOperations.contactId and max contactOperations.date.
aggregate([{$unwind : "$contactOperations"}, {$group : {"_id":{"accountId":"$accountId", "contactId":"$contactOperations.contactId"}, "date":{$max:"$contactOperations.date"} }}])
The result I get is:
"_id" : { "accountId" : "123", "contactId" : "2" }, "time" : 502 }
"_id" : { "accountId" : "123", "contactId" : "1" }, "time" : 501 }
Which seems correct so far, but I also need the contactOperations.operation field that was recorded with $max date. How can I select that?

Answer 1

You have to sort the unwind values then apply $last operator to get operation for max date. Hope this query will solve your problem.
aggregate([
{
$unwind: "$contactOperations"
},
{
$sort: {
"date": 1
}
},
{
$group: {
"_id": {
"accountId": "$accountId",
"contactId": "$contactOperations.contactId"
},
"date": {
$max: "$contactOperations.date"
},
"operationId": {
$last: "$contactOperations.operation"
}
}
}
])