Post and Comments Mapping

Hello,
I need to create a mapping for a forum website, where you have Post (with some properties) and each Post can have multiple comment (with more or less same properties).
For this scenario is it better to have

  • Nested Object
  • Parent child Documents
  • Flat mapping with a field to determine if it is post or comment with the post_id

Thanks in advance

is there any answer?

I'd prefer the later.