[{"name":"pencil","purchase_record":{"1":"$900","2":"$1000","3":"$1100","4":"$1200"}},{"name":"pen","purchase_record":{"1":"$1000","2":"$1200","3":"$900","4":"$1100","5":"$1100"}}]

db.collection.aggregate([{$project:{_id:0,name:1,purchaseCount:{$size:{"$objectToArray":"$purchase_record"}}}}])