How to get length of sub array in mongo query - mongodb

Question

Below is the sample data:
db.infos.find()
{
"groupId": "1111",
"customerId": "A100",
"tracks": [{
"trackId": "234",
"infos": [{
"location": {
"address": "street1",
"city": "test",
"country": "US"
}
},
{
"location": {
"address": "street2",
"city": "test",
"country": "US"
}
},
{
"location": {
"address": "street3",
"city": "test",
"country": "US"
}
}
]
}]
}
{
"groupId": "2222",
"customerId": "A100",
"tracks": [{
"trackId": "345",
"infos": [{
"location": {
"address": "street4",
"city": "test",
"country": "US"
}
},
{
"location": {
"address": "street5",
"city": "test",
"country": "US"
}
},
{
"location": {
"address": "street5",
"city": "test",
"country": "US"
}
}
]
}]
}
{
"groupId": "2222",
"customerId": "A100",
"tracks": [{
"trackId": "666",
"infos": [{
"location": {
"address": "street4",
"city": "test",
"country": "US"
}
},
{
"location": {
"address": "street5",
"city": "test",
"country": "US"
}
},
{
"location": {
"address": "street5",
"city": "test",
"country": "US"
}
}
]
}]
}
We need a query to get the length of "infos" sub array at groupId level. In the above sample data, we need the below output:
1111, 3
2222, 6
Tried below, but its not working:
db.infos.aggregate([{"$project": {"groupId": "$groupId", "samples": "$tracks.infos"}}, {"$group": {"_id": "$groupId", "samples": {"$push": "$samples"}}}, {"$project": {"_id": 1, "samples": {"$size": "$samples"}}}], { "allowDiskUse": true });
Also with large amount of data, above query is throwing an ExceededMemoryLimit exception:
2021-07-10T07:20:14.415+0000 E QUERY [js] Error: command failed: {
"ok" : 0,
"errmsg" : "$push used too much memory and cannot spill to disk. Memory limit: 104857600 bytes",
"code" : 146,
"codeName" : "ExceededMemoryLimit"
} : aggregate failed :
_getErrorWithCode#src/mongo/shell/utils.js:25:13
doassert#src/mongo/shell/assert.js:18:14
_assertCommandWorked#src/mongo/shell/assert.js:580:17
assert.commandWorked#src/mongo/shell/assert.js:673:16
DB.prototype._runAggregate#src/mongo/shell/db.js:260:9
DBCollection.prototype.aggregate#src/mongo/shell/collection.js:1074:12
#(shell):1:1

Answer 1

This should work with unwind:
db.collection.aggregate([
{
$unwind: "$tracks"
},
{
$group: {
"_id": "$groupId",
"count": {
"$sum": {
"$size": "$tracks.infos"
}
}
}
}
])
Even if your tracks array has more than one object this will add them up. Playground