Structure with more Depth vs more breadth in Mongodb overhead

Question

I'm trying to store user notifications in mongoDB ( My main database is Mysql ) and the notification table has over 2,500,000 records for each notification.

I want to know that should i store each notification in one row or store them based on user_id, each row containing all notifications of an user. Queries that will perform on these data are selecting all notifications of a user and selecting all notification with notified flag set to false and updating a flag of a notification with notification id.

Each row on table has id,user_id,description,read,notified

Which one has less overhead?

Answer 1

Store each notification in one row would be recommended since query or update would be easier.

Good

_id:"1"
user_id: "1",
description: "123",
read: true

Only if notifications and fields are less each user.

_id:"1",
user_id: "1",
notifications: [ 
{
  _id:"1",
  description: "123",
  read: true
}]