weird behaviour mongo aggregation framework - mongodb

Question

I have one Document called account holding and it has below records,
{ "_id" : ObjectId("57cfbb09e4b024be2f1bce57"),
"_class" : "com.commercestudio.domain.AccountHolding",
"accountId" : "5732933ae4b0b709443b0d1e",
"companyId" : "57223d6de4b06c4ef00415b5",
"brokerageAccountId" : "5KC05007",
"symbol" : "AGG",
"quantity" : 1.0,
"pricePaid" : 112.55,
"processDate" : ISODate("2016-09-06T00:00:00.000Z"),
"recordDate" : ISODate("2016-09-06T00:00:00.000Z"),
"createdOn" : ISODate("2016-09-07T07:00:25.479Z")
}
{ "_id" : ObjectId("57cfbb09e4b024be2f1bce5b"),
"_class" : "com.commercestudio.domain.AccountHolding",
"accountId" : "5732933ae4b0b709443b0d1e",
"companyId" : "57223d6de4b06c4ef00415b5",
"brokerageAccountId" : "5KC05007",
"symbol" : "LQD",
"quantity" : 4.0,
"pricePaid" : 123.78,
"processDate" : ISODate("2016-09-06T00:00:00.000Z"),
"recordDate" : ISODate("2016-09-06T00:00:00.000Z"),
"createdOn" : ISODate("2016-09-07T07:00:25.498Z")
}
.....
now I apply aggrigration framework for finding out latest record date data for perticulat accountId,
db.accountHolding.aggregate(
[
{
"$match": {
"accountId": "5834caf32ae7bacc527ef2f3",
"symbol": {
"$in": [
"IUSG",
"VEA",
"IEMG",
"SCHX",
"VBR",
"IUSV",
"VOE"
]
}
}
},
{
"$group": {
"_id": "$symbol",
"recordDate": {
"$last": "$recordDate"
},
"quantity": {
"$last": "$quantity"
},
"pricePaid": {
"$last": "$pricePaid"
}
}
}
])
and it returns two different results in two different environments,
On my development env. it shows,
{
"_id" : "VEA",
"recordDate" : ISODate("2018-03-02T00:00:00.000Z"),
"quantity" : 22.79609, "pricePaid" : 44.14
}
{ "_id" : "IUSG",
"recordDate" : ISODate("2018-03-02T00:00:00.000Z"),
"quantity" : 8.87831,
"pricePaid" : 55.79
}
something like this and from production env. it shows,
{
"_id" : "VEA",
"recordDate" : ISODate("2018-02-26T00:00:00Z"),
"quantity" : 22.79609,
"pricePaid" : 45.76
}
{
"_id" : "IUSG",
"recordDate" : ISODate("2018-02-26T00:00:00Z"),
"quantity" : 8.87831,
"pricePaid" : 57.47
}
actually, I am unable to find out the solution why this weird behaviour is taken place, as both env has same data.
My database server is deployed on AWS instance.
Can someone help me out with finding out the root cause and solution for the same?

Answer 1

This is expected behavior.
From the docs,
Returns the value that results from applying an expression to the last
document in a group of documents that share the same group by a field.
Only meaningful when documents are in a defined order.
Add $sort before $group stage.
{$sort:{recordDate:1}}