[{"value":"field2","field1":[],"field2":[{id:5}]}]

db.collection.aggregate([{$addFields:{"values":{$arrayElemAt:[{$filter:{input:{$objectToArray:"$$ROOT"},as:"field",cond:{$eq:["$$field.k","$value"]}}},0]}}},{$unwind:"$values.v"},{$replaceRoot:{newRoot:"$values.v"}},])