[{"_id":ObjectId("61ea385114e22e000c58a502"),"stu_id":"h224566378","stu_inf":[{"_id":ObjectId("61ea3a35c34a334514e7e8df"),"stu_id":"h224566378","gender":"male","class":"A",}]}]

db.collection.aggregate([{$project:{"stu_id":1,"stu_class":{$first:"$stu_inf.class"}}}])