[{"_id":"main1","title":"First","code":"C1","subDoc":{"active":true,"sub_id":"main1sub1","order":1}},{"_id":"main2","title":"Second","code":"C2","subDoc":{"active":true,"sub_id":"main2sub1","order":1}},{"_id":"main3","title":"Third","code":"C3","subDoc":{"active":false,"sub_id":"main3sub1","order":1}}]

db.collection.aggregate([{$match:{code:{"$lt":"C2"}}},{"$sort":{code:-1}},{$limit:1}])