MongoDB data grouping with aggregation

Question

I have data collection like this:

> db.LogBuff.find()
{ "_id" : ObjectId("578899d5d2b76f77d083f16c"), "SUBJECT" : "DD", "SYS" : "A" }
{ "_id" : ObjectId("578899d5d2b76f77d083f16d"), "SUBJECT" : "AA", "SYS" : "B" }
{ "_id" : ObjectId("578899d5d2b76f77d083f16e"), "SUBJECT" : "BB", "SYS" : "A" }
{ "_id" : ObjectId("578899d5d2b76f77d083f16f"), "SUBJECT" : "AA", "SYS" : "C" }
{ "_id" : ObjectId("578899d5d2b76f77d083f170"), "SUBJECT" : "BB", "SYS" : "A" }
{ "_id" : ObjectId("578899d5d2b76f77d083f171"), "SUBJECT" : "BB", "SYS" : "A" }
{ "_id" : ObjectId("578899d5d2b76f77d083f172"), "SUBJECT" : "CC", "SYS" : "B" }

And I want to extract output as below (with distinct "SYS" values)

{"SUBJECT" : "AA", "SYS" : ["A","B","C","D"]}
{"SUBJECT" : "BB", "SYS" : ["A","B","C","D"]}
{"SUBJECT" : "CC", "SYS" : ["A","B","C"]}

Here is my code and I am stuck in the middle, please help me to sort this

db.LogBuff.aggregate([{
     "$unwind": "$SYS"
}, {
     "$group": {
             _id: {
                     "_id": "$SUBJECT"
             },
             SYST: {
                     $addToSet: "$SYS"
             }
     }
}, {
     "$unwind": "$SYST"
}, {
     "$group": {
             _id: {
             "SUBJECT": "$_id",
                     "SYST":"$SYST"
             }
     }
}])

Answer 1

Just group by the _id, and addToSet the SYS values:

db.LogBuff.aggregate([
    {
     "$group": {
             _id: {
                     "_id": "$SUBJECT"
             },
             SYST: {
                     $addToSet: "$SYS"
             }
        }
    }
])

No need for unwind, one group should get you the desired result.

Result of group aggregation on your example data:

{ "_id" : { "_id" : "CC" }, "SYST" : [ "B" ] }
{ "_id" : { "_id" : "BB" }, "SYST" : [ "A" ] }
{ "_id" : { "_id" : "AA" }, "SYST" : [ "C", "B" ] }
{ "_id" : { "_id" : "DD" }, "SYST" : [ "A" ] }