[{"_id":ObjectId("5e4ff8357f8bc30a75155c3f"),"id":123,"name":"sain","classes":[{"c_id":1,"c_name":"1(one)","c_strn":"100","students":[{"s_name":"Leo","s_roll":"17","s_address":"strt24"},{"s_name":"Kyle","s_roll":"18","s_address":"strt24"}]},{"c_id":2,"c_name":"2(one)","c_strn":"100","students":[{"s_name":"Leo2","s_roll":"17","s_address":"strt241"},{"s_name":"Kyle2","s_roll":"18","s_address":"strt24"}]},{"c_id":3,"c_name":"3(one)","c_strn":"100","students":[{"s_name":"Leo3","s_roll":"17","s_address":"strt24"},{"s_name":"Kyle3","s_roll":"18","s_address":"strt24"}]}]},{"_id":ObjectId("5e4ff83d7f8bc30a75155cfd"),"id":456,"name":"sain","classes":[{"c_id":1,"c_name":"1(one)","c_strn":"100","students":[{"s_name":"Leo","s_roll":"17","s_address":"strt24"},{"s_name":"Kyle","s_roll":"18","s_address":"strt24"}]},{"c_id":2,"c_name":"2(one)","c_strn":"100","students":[{"s_name":"Leo2","s_roll":"17","s_address":"strt241"},{"s_name":"Kyle2","s_roll":"18","s_address":"strt24"}]},{"c_id":3,"c_name":"3(one)","c_strn":"100","students":[{"s_name":"Leo3","s_roll":"17","s_address":"strt24"},{"s_name":"Kyle3","s_roll":"18","s_address":"strt24"}]}]}]

db.collection.aggregate([{$match:{id:123}},{$unwind:"$classes"},{$match:{"classes.c_id":1}},{$project:{_id:0,students:"$classes.students"}}])