[{"_id":"Egg shop","records":[{"_id":1,"price":22},{"_id":2,"price":18},{"_id":3,"price":34},{"_id":4,"price":31},{"_id":5,"price":13},]}]

db.collection.aggregate([{"$addFields":{"records":{"$map":{"input":"$records","as":"r","in":{"_id":"$$r._id","price":"$$r.price","below20":{$lt:["$$r.price",20]},"over30":{$gt:["$$r.price",30]}}}}}},{"$addFields":{"lowestCheapIndex":{"$indexOfArray":["$records.below20",true]},"highestExpensiveIndex":{"$subtract":[{"$subtract":[{$size:"$records"},{"$indexOfArray":[{"$reverseArray":"$records.over30"},true]}]},1]}}}])