Database
[{levelName:"Level 1",levelNames:{"Level 1":"5e567993b6ed4b7b4d2c044d","Level 2":"5e567996a7826d45f836dfa3","Level 3":"5e5679991f515a01c73e9006",}}]
Query
db.collection.aggregate([{$set:{data:{$objectToArray:"$levelNames"}}},{$set:{currentLevelId:{$arrayElemAt:["$data.v",{$indexOfArray:["$data.k","$levelName"]}]}}},{$unset:"data"}])