How to limit the no of columns in output while doing aggregate operation in Mongo DB - mongodb

Question

My function looks like below.
function (x)
{
var SO2Min = db.AirPollution.aggregate(
[
{
$match : {"SO2":{$ne:'NA'}, "State":{$eq: x} }
},
{
$group:
{
_id: x,
SO2MinQuantity: { $min: "$SO2" }
}
},
{
$project:
{SO2MinQuantity: '$SO2MinQuantity'
}
}
]
)
db.AirPollution.update
(
{
"State": "West Bengal"},
{
$set: {
"MaxSO2": SO2Max
}
},
{
"multi": true
}
);
}
Here, AirPolltuion is my Collection. If I run this function, the collection gets updated with new column MaxSO2 as below.
{
"_id" : ObjectId("5860a2237796484df5656e0c"),
"Stn Code" : 11,
"Sampling Date" : "02/01/15",
"State" : "West Bengal",
"City/Town/Village/Area" : "Howrah",
"Location of Monitoring Station" : "Bator, Howrah",
"Agency" : "West Bengal State Pollution Control Board",
"Type of Location" : "Residential, Rural and other Areas",
"SO2" : 10,
"NO2" : 40,
"RSPM/PM10" : 138,
"PM 2.5" : 83,
"MaxSO2" : {
"_batch" : [
{
"_id" : "West Bengal",
"SO2MaxQuantity" : 153
}
],
"_cursor" : {}
}
}
Where we can see, that MaxSO2 has been added as a sub document. But I want that column to be added inside same document as a field, not as a part of sub document. Precisely, I dont want batch and cursor fields to come up. Please help.

Answer 1

Since the aggregate function returns a cursor, you can use the toArray() method which returns an array that contains all the documents from a cursor and then access the aggregated field. Because you are returning a single value from the aggregate, there's no need to iterate the results array, just access the first and only single document in the result to get the value.
Once you get this value you can then update your collection using updateMany() method. So you can refactor your code to:
function updateMinAndMax(x) {
var results = db.AirPollution.aggregate([
{
"$match" : {
"SO2": { "$ne": 'NA' },
"State": { "$eq": x }
}
},
{
"$group": {
"_id": x,
"SO2MinQuantity": { "$min": "$SO2" },
"SO2MaxQuantity": { "$max": "$SO2" }
}
},
]).toArray();
var SO2Min = results[0]["SO2MinQuantity"];
var SO2Max = results[0]["SO2MaxQuantity"];
db.AirPollution.updateMany(
{ "State": x },
{ "$set": { "SO2MinQuantity": SO2Min, "SO2MaxQuantity": SO2Max } },
);
}
updateMinAndMax("West Bengal");