How to store comments in NoSQL (MongoDB)?

Question

1 way

Comments are embedded in Post document:

{
  "_id": ObjectId(12345),
  "title": "Cat ate elephant!",
  "body": "Lorem ipsum dolor sit amet, consectetur adipiscing elit. Aenean convallis pretium vulputate.",
  "comments": [
    {"name": "Cat", "body": "MEOW!"},
    {"name": "Elephant", "body": "I was eaten by cat, lol!"},
    {"name": "Human", "body": "I am hungry!"}
  ]
}

---

2 way

Relationship between Post and Comments (in separate documents). Post has many Comments:

// POST //
{
  "_id": ObjectId(12345),
  "title": "Cat ate elephant!"
  "body": "Lorem ipsum dolor sit amet, consectetur adipiscing elit. Aenean convallis pretium vulputate."
}


// Comments //

{
  "_id": ObjectId(...),
  "post_id": ObjectId(12345),
  "name": "Cat",
  "body": "MEOW!"
}

{
  "_id": ObjectId(...),
  "post_id": ObjectId(12345),
  "name": "Elephant",
  "body": "I was eaten by cat, lol!"
}

{
  "_id": ObjectId(...),
  "post_id": ObjectId(12345),
  "name": "Human",
  "body": "I am hungry!"
}

---

Which way is better?

Answer 1

Method 1

• No need to joins => fast access to data
• The NoSQL way of doing it
• Every comment is just related to just that post, so why not store them together(you store the title and the body together ;)

If you have large documents, >15.5 Megabyte, and you gets a loot of comments then you probably will need to store them somewhere else. This because the maximum document size is 16 Megabyte.

Method 2 is the RDMBS way of doing it, Mongo does not have joins inbuilt so you will need to do them in your application.

Answer 2

The first way is preferred, as long as the document is not write-heavy. If you're going to have 5000 comments added to a post within a minute, then use the second method.