Database
[{"_id":1,"username":"test","email":"t@t","password":"","submits":[{"date":"16/10/2020","subjects":[{"subject1":{"time":"","description":""}}]}]}]
Query
db.collection.update({"username":"test","submits.date":"16/10/2020"},{$push:{"submits.$.subjects":{"subjectName":{"time":"1234","description":"desc"}}}})