Database
[{"_id":"L","count":2.0},{"_id":"XL","count":2.0},{"_id":"XXXL","count":1.0},{"_id":"M","count":1.0}]
Query
db.collection.aggregate([{"$group":{"_id":null,"array":{"$push":{"k":"$_id","v":"$count"}}}},{"$replaceRoot":{"newRoot":{"$arrayToObject":"$array"}}},{"$addFields":{"S":{"$ifNull":["$S",0]},"M":{"$ifNull":["$M",0]},"L":{"$ifNull":["$L",0]},"XL":{"$ifNull":["$XL",0]},"XXL":{"$ifNull":["$XXL",0]},"XXXL":{"$ifNull":["$XXXL",0]}}}])