Database
[{MMSI:1,"key":1},{MMSI:1,"key":2},{MMSI:2,"key":3},{MMSI:2,"key":4}]
Query
db.collection.aggregate({"$group":{"_id":"$MMSI","a":{"$push":"$$ROOT"}}},{"$replaceRoot":{"newRoot":{$arrayToObject:{$map:{input:["$$ROOT"],as:"el",in:{"k":{$toString:"$$el._id"},"v":"$$el.a"}}}}}})