[{"_id":"17202155","completed":["cse331","cse312"],"incompleted":[{"name":"math","preReq":[]},{"name":"science","preReq":["bang","eng","math"]},{"name":"astronomy","preReq":["bang","eng","science"]}]},{"_id":"17202157","completed":["cse331","cse312"],"incompleted":[{"name":"math","preReq":[]},{"name":"science","preReq":[]},{"name":"astronomy","preReq":["bang","eng","science"]}]}]

db.collection.aggregate([{$unwind:"$incompleted"},{$match:{"incompleted.preReq":[]}},{$group:{_id:"$_id",incompleted:{$push:"$incompleted"}}}])