Database
[{"item1":{"a":1},"item2":{"a":2}},{"item1":{"a":3},"item3":{"a":4}},{"item1":{"a":5},"item2":{"a":6}},]
Query
db.collection.aggregate([{$project:{root:{$objectToArray:"$$ROOT"}}},{$unwind:"$root"},{$group:{_id:"$root.k",total:{$sum:1}}},{$group:{_id:null,obj:{$push:{k:"$_id",v:"$total"}}}},{$replaceRoot:{newRoot:{$arrayToObject:"$obj"}}},{$project:{_id:0}}])