[{"_id":1,"rxid":1234,"rx1234":[{"A":"somevalue","B":"someothervalue",},{"A":"somevalue2",}]},{"_id":2,"rxid":2345}]

db.collection.aggregate([{"$project":{"concatValue":{"$concat":["rx",{"$toString":"$rxid"}]},"data":{"$objectToArray":"$$ROOT"}}},{$unwind:"$data"},{$match:{$and:[{$expr:{"$eq":["$concatValue","$data.k"]}}]}},{"$project":{"data.v.A":1,"data.v.B":1}}])