db={"coll":[{"_id":"abc","field1":[],"field2":"somVal","field3":[{"extrafield":"afield","fieldToSort":"bcdef"},{"fieldToSort":"abcde"}]},{"_id":"def","field1":[{"fieldToSort":"bcdef"},{"fieldToSort":"abcde"}],"field3":[]}],"dummy":[{}]}

db.coll.aggregate([{"$lookup":{"from":"dummy","let":{"field1":"$field1"},"pipeline":[{"$set":{"field1":"$$field1"}},{"$unwind":{"path":"$field1"}},{"$sort":{"field1.fieldToSort":1}},{"$group":{"_id":null,"field1":{"$push":"$field1"}}}],"as":"field1"}},{"$set":{"field1":{"$cond":[{"$eq":["$field1",[]]},[],{"$arrayElemAt":["$field1.field1",0]}]}}},{"$lookup":{"from":"dummy","let":{"field3":"$field3"},"pipeline":[{"$set":{"field3":"$$field3"}},{"$unwind":{"path":"$field3"}},{"$sort":{"field3.fieldToSort":1}},{"$group":{"_id":null,"field3":{"$push":"$field3"}}}],"as":"field3"}},{"$set":{"field3":{"$cond":[{"$eq":["$field3",[]]},[],{"$arrayElemAt":["$field3.field3",0]}]}}}])