MongoDB: Sort in combination with Aggregation group - mongodb

Question

I have a collection called transaction with below documents,
/* 0 */
{
"_id" : ObjectId("5603fad216e90d53d6795131"),
"statusId" : "65c719e6727d",
"relatedWith" : "65c719e67267",
"status" : "A",
"userId" : "100",
"createdTs" : ISODate("2015-09-24T13:15:36.609Z")
}
/* 1 */
{
"_id" : ObjectId("5603fad216e90d53d6795134"),
"statusId" : "65c719e6727d",
"relatedWith" : "65c719e6726d",
"status" : "B",
"userId" : "100",
"createdTs" : ISODate("2015-09-24T13:14:31.609Z")
}
/* 2 */
{
"_id" : ObjectId("5603fad216e90d53d679512e"),
"statusId" : "65c719e6727d",
"relatedWith" : "65c719e6726d",
"status" : "C",
"userId" : "100",
"createdTs" : ISODate("2015-09-24T13:13:36.609Z")
}
/* 3 */
{
"_id" : ObjectId("5603fad216e90d53d6795132"),
"statusId" : "65c719e6727d",
"relatedWith" : "65c719e6726d",
"status" : "D",
"userId" : "100",
"createdTs" : ISODate("2015-09-24T13:16:36.609Z")
}
When I run the below Aggregation query without $group,
db.transaction.aggregate([
{
"$match": {
"userId": "100",
"statusId": "65c719e6727d"
}
},
{
"$sort": {
"createdTs": -1
}
}
])
I get the result in expected sorting order. i.e Sort createdTs in descending order (Minimal result)
/* 0 */
{
"result" : [
{
"_id" : ObjectId("5603fad216e90d53d6795132"),
"createdTs" : ISODate("2015-09-24T13:16:36.609Z")
},
{
"_id" : ObjectId("5603fad216e90d53d6795131"),
"createdTs" : ISODate("2015-09-24T13:15:36.609Z")
},
{
"_id" : ObjectId("5603fad216e90d53d6795134"),
"createdTs" : ISODate("2015-09-24T13:14:31.609Z")
},
{
"_id" : ObjectId("5603fad216e90d53d679512e"),
"createdTs" : ISODate("2015-09-24T13:13:36.609Z")
}
],
"ok" : 1
}
If I apply the below aggregation with $group, the resultant is inversely sorted(i.e Ascending sort)
db.transaction.aggregate([
{
"$match": {
"userId": "100",
"statusId": "65c719e6727d"
}
},
{
"$sort": {
"createdTs": -1
}
},
{
$group: {
"_id": {
"statusId": "$statusId",
"relatedWith": "$relatedWith",
"status": "$status"
},
"status": {$first: "$status"},
"statusId": {$first: "$statusId"},
"relatedWith": {$first: "$relatedWith"},
"createdTs": {$first: "$createdTs"}
}
}
]);
I get the result in inverse Order i.e. ** Sort createdTs in Ascending order**
/* 0 */
{
"result" : [
{
"_id" : ObjectId("5603fad216e90d53d679512e"),
"createdTs" : ISODate("2015-09-24T13:13:36.609Z")
},
{
"_id" : ObjectId("5603fad216e90d53d6795134"),
"createdTs" : ISODate("2015-09-24T13:14:31.609Z")
},
{
"_id" : ObjectId("5603fad216e90d53d6795131"),
"createdTs" : ISODate("2015-09-24T13:15:36.609Z")
},
{
"_id" : ObjectId("5603fad216e90d53d6795132"),
"createdTs" : ISODate("2015-09-24T13:16:36.609Z")
}
],
"ok" : 1
}
Where am I wrong ?

Answer 1

The $group stage doesn't insure the ordering of the results. See here the first paragraph.
If you want the results to be sorted after a $group, you need to add a $sort after the $group stage.
In your case, you should move the $sort after the $group and before you ask the question : No, the $sort won't be able to use an index after the $group like it does before the $group :-).
The internal algorithm of $group seems to keep some sort of ordering (reversed apparently), but I would not count on that and add a $sort.

Answer 2

You are not doing anything wrong here, Its a $group behavior in Mongodb
Lets have a look in this example
Suppose you have following doc in collection
{ "_id" : 1, "item" : "abc", "price" : 10, "quantity" : 2, "date" : ISODate("2014-01-01T08:00:00Z") }
{ "_id" : 2, "item" : "jkl", "price" : 20, "quantity" : 1, "date" : ISODate("2014-02-03T09:00:00Z") }
{ "_id" : 3, "item" : "xyz", "price" : 5, "quantity" : 5, "date" : ISODate("2014-02-03T09:05:00Z") }
{ "_id" : 4, "item" : "abc", "price" : 10, "quantity" : 10, "date" : ISODate("2014-02-15T08:00:00Z") }
{ "_id" : 5, "item" : "xyz", "price" : 5, "quantity" : 10, "date" : ISODate("2014-02-15T09:05:00Z") }
{ "_id" : 6, "item" : "xyz", "price" : 5, "quantity" : 5, "date" : ISODate("2014-02-15T12:05:10Z") }
{ "_id" : 7, "item" : "xyz", "price" : 5, "quantity" : 10, "date" : ISODate("2014-02-15T14:12:12Z") }
Now if you run this
db.collection.aggregate([{ $sort: { item: 1,date:1}} ] )
the output will be in ascending order of item and date.
Now if you add group stage in aggregation pipeline it will reverse the order.
db.collection.aggregate([{ $sort: { item: 1,date:1}},{$group:{_id:"$item"}} ] )
Output will be
{ "_id" : "xyz" }
{ "_id" : "jkl" }
{ "_id" : "abc" }
Now the solution for your problem
change "createdTs": -1 to "createdTs": 1 for group