How to calculate a sum of specific documents using aggregation? - mongodb

Question

I have a schema representing a message thread. So each document in the mongo database looks something like:
{
id: "thread_id",
participants: ["user1", "user2"],
unReadMessageCounts: [
{
participant: "user1",
count: 5
},
{
participant: "user2",
count: 3
}
}
What I want to do is get a sum of all unread messages counts for a given user - say, "user2". I know I could do this by just doing a find() on the collection and then writing a function to sum up to counts for a given user. But I'd like to use mongo's aggregate functionality if possible. I know I can do a match to first select all threads in which "user2" is a participant, but then how do I construct the group and/or sum expressions to pull out the right field from the document?

Answer 1

Use the following aggregation pipeline to get the desired result. The initial step filters out the incoming documents to only accept "user2" participant by way of the $match operator.
The preceding pipeline stage then "denormalizes" the unReadMessageCounts array through the $unwind operator that outputs 2 documents from the array for each incumbent document (in your above sample data).
Further filtering is necessary to aggregate data for the correct participant and this is done through another $match pipeline step.
The final aggregation operation using $group specifies a group _id of null, calculating the total counts for all documents in the pipeline using the accumulator operator $sum on the "unReadMessageCounts.count" field.
So, running this aggregation pipeline on the sample data given:
db.collection.aggregate([
{
"$match": { "unReadMessageCounts.participant": "user2" }
},
{ "$unwind" : "$unReadMessageCounts" },
{
"$match": { "unReadMessageCounts.participant": "user2" }
},
{
"$group": {
"_id": null,
"total": { "$sum": "$unReadMessageCounts.count" }
}
}
])
will yield the result:
/* 0 */
{
"result" : [
{
"_id" : null,
"total" : 3
}
],
"ok" : 1
}

Answer 2

You can use the $redact operator to as shown here to limit the size of documents to process in the pipeline then you $unwind your documents and in the $group stage you use the $sum accumulator operator to return total of unread message for "user2".
db.collection.aggregate([
{ "$match": {
"unReadMessageCounts": {
"$elemMatch": { "participant": "user2" }
}
}},
{ "$redact": {
"$cond": [
{ "$or": [
{ "$eq": [ "$participant", "user2" ] },
{ "$not" : "$participant" }
]},
"$$DESCEND",
"$$PRUNE"
]
}},
{ "$unwind": "$unReadMessageCounts" },
{ "$group": {
"_id": null,
"total": { "$sum": "$unReadMessageCounts.count" }
}}
])