[{id:1,book:"Flash",chapters:[{chap_no:"1",sub_chapter:[{sub_no:1,description:"<description>"},{sub_no:2,description:"<description>"},]}]}]

db.collection.aggregate([{"$match":{"chapters.sub_chapter.sub_no":2}},{"$unwind":"$chapters"},{"$unwind":"$chapters.sub_chapter"},{"$match":{"chapters.sub_chapter.sub_no":2}},{"$replaceRoot":{"newRoot":"$chapters.sub_chapter"}}])