Database
[{_id:"5bd258a7877e74059b6b65b2",year:2017,title:"One"},{_id:"5bd258a7877e74059b6b65b3",year:2017,title:"Two"},{_id:"5bd258a7877e74059b6b65b4",year:2018,title:"Three"},{_id:"5bd258a7877e74059b6b65b5",year:2018,title:"Four"}]
Query
db.collection.aggregate([{$group:{_id:"$year",docs:{$addToSet:"$$CURRENT"}}},{"$group":{"_id":null,"data":{"$push":{"k":{$toString:"$_id"},"v":"$docs"}}}},{"$replaceRoot":{"newRoot":{"$arrayToObject":"$data"}}}])