How to combine results in a Mongo aggregation query - mongodb

Question

I'm new to aggregation queries in Mongo and been really struggling trying to produce the output I want. I have the following aggregation query:
db.events.aggregate([
{ $match: { requestState: "APPROVED" } },
{ $unwind: { path: "$payload.description" } },
{ $group: { _id: { instr: "$payload.description", bu: "$createdByUser", count: { $sum: 1 } } } }
]);
that returns the following results:
{ "_id" : { "instr" : "ABC-123", "bu" : "BU2", "count" : 1 } }
{ "_id" : { "instr" : "ABC-123", "bu" : "BU1", "count" : 1 } }
{ "_id" : { "instr" : "DEF-456", "bu" : "BU1", "count" : 1 } }
How can I amend the aggregation query so that there are only 2 documents returned instead of 3? With the two "ABC-123" results combined into a single result with a new array of counts with the "bu" and "count" fields i.e.
{ "_id" : { "instr" : "ABC-123", "counts": [ { "bu" : "BU1", "count" : 1 }, { "bu" : "BU2", "count" : 1 } ] } }
Many thanks

Answer 1

You can add another stage to only $group by _id.instr and another stage to $project to your desired output shape
db.events.aggregate([
{
$match: { requestState: "APPROVED" }
},
{
$unwind: { path: "$payload.description" }
},
{
$group: {
_id: { instr: "$payload.description", bu: "$createdByUser", count: { $sum: 1 } }
}
},
{
$group: {
_id: { instr: "$_id.instr" },
counts: { $push: { bu: "$_id.bu", count: "$_id.count" } }
}
},
{
$project: {
_id: { instr: "$_id.instr", counts: "$counts" }
}
}
]);