Is there a way to group results from multiple documents when performing aggregation - mongodb

Question

I am new to mongo and trying to perform aggregation query to calculate min/max of timestamps for a given document.
Sample documents are below -
{
"_id" : ObjectId("5c9cd93adddca9ebb2b3fcba"),
"frequency" : 5,
"s_id" : "30081993",
"timestamp" : NumberLong(1546300800000),
"date" : ISODate("2019-01-01T00:00:00.000Z"),
"values" : {
"1547439900000" : {
"number_of_values" : 3,
"min_value" : 32.13,
"max_value" : 81.42
},
"1547440200000" : {
"number_of_values" : 3,
"min_value" : 48.08,
"max_value" : 84.52
},
"1547440500000" : {
"number_of_values" : 2,
"min_value" : 27.39,
"max_value" : 94.64
}
}
}
{
"_id" : ObjectId("5c9cd851dddca9ebb2b3f2ac"),
"frequency" : 5,
"s_id" : "27061995",
"timestamp" : NumberLong(1546300800000),
"date" : ISODate("2019-01-01T00:00:00.000Z"),
"values" : {
"1547539900000" : {
"number_of_values" : 31,
"min_value" : 322.13,
"max_value" : 831.42
},
"1547540200000" : {
"number_of_values" : 3,
"min_value" : 418.08,
"max_value" : 8114.52
},
"1547740500000" : {
"number_of_values" : 2,
"min_value" : 207.39,
"max_value" : 940.64
}
}
}
I have come up with the following query which works for a single document.
db.testdb.aggregate([
{
$match: {
"s_id": "30081993",
"frequency": 5,
}
},
{
$project: {
_id: 1,
valuesarray: {
$objectToArray: "$values"
}
}
},
{
$unwind: "$valuesarray"
},
{
$group: {
"_id": "",
"min_timestamp": {
$min: "$valuesarray.k"
},
"max_timestamp": {
$max: "$valuesarray.k"
}
}
}
]);
The output is below
{
"_id" : "",
"min_timestamp" : "1547439900000",
"max_timestamp" : "1547440500000"
}
I want an aggregation query which can calculate the max/min of timestamps but for multiple documents i.e I want to use a $in operator during the $match stage and get min/max of all s_id. Is this possible?
Expected :
{
"_id" : "30081993",
"min_timestamp" : "1547439900000",
"max_timestamp" : "1547440500000"
}
{
"_id" : "27061995",
"min_timestamp" : "1547539900000",
"max_timestamp" : "1547740500000"
}

Answer 1

Yes, only small changes are required to make this work for multiple documents.
In $match stage, specify your $in query:
$match: {
"s_id": { $in : [ "30081993", "27061995" ] },
"frequency": 5,
}
In $project stage, rename s_id to _id, to ensure we keep the s_id associated with each document:
$project: {
_id: "$s_id",
valuesarray: {
$objectToArray: "$values"
}
}
In $group stage, group by _id (originally s_id), to ensure we correctly group the timestamps together before calculating $min/$max:
$group: {
"_id": "$_id",
"min_timestamp": {
$min: "$valuesarray.k"
},
"max_timestamp": {
$max: "$valuesarray.k"
}
}
Whole pipeline:
db.testdb.aggregate([
{
$match: {
"s_id": { $in : [ "30081993", "27061995" ] },
"frequency": 5,
}
},
{
$project: {
_id: "$s_id",
valuesarray: {
$objectToArray: "$values"
}
}
},
{
$unwind: "$valuesarray"
},
{
$group: {
"_id": "$_id",
"min_timestamp": {
$min: "$valuesarray.k"
},
"max_timestamp": {
$max: "$valuesarray.k"
}
}
}
]);