[{_id:123123,name:"abc",topic:{dasfdsf:"Z"}},{_id:123124,name:"abc",topic:{sdfgff:"P"}},{_id:123125,name:"abc",topic:{sdff:"A"}}]

db.collection.aggregate([{$addFields:{topicArr:{$objectToArray:"$topic"}}},{$sort:{"topicArr.v":1}},{$unset:"topicArr"}])