Database
[{_id:ObjectId("5fa917baffdd553806e96ca4"),priority:1,fields:{1:{id:NumberInt("1"),value:NumberInt("1"),label:"Din 1",dimension:null,valueHuman:"1"},9:{id:NumberInt("9"),value:NumberInt("259"),label:"",dimension:"",valueHuman:""}}}]
Query
db.collection.aggregate([{$addFields:{fields:{$arrayToObject:{$filter:{input:{$objectToArray:"$fields"},cond:{$eq:[{$toInt:"$$this.k"},"$priority"]}}}}}}])