Mongodb Post/Comment use case
The correct answer depends on a few application design decisions.

Reasons why you should embed Comments in Posts:


You have a limit on the number of comments that can be on a post
The comment limit is relatively small (mongodb docs have a hard limit
of 4mb but you don't want to be pulling 4mb data down at once, so
your limit should take this into consideration)
You need access to all comments on a post when you load a post


Reasons why you should reference Posts from Comments:


You want to have unlimited comments on a post
You're ok with loading the post first then going back to the database every time you need comments
You want to fetch comments without posts


I offer a combined approach. Store comments in their own collection but store a total comments counter and possibly recent comments against the post. This way you can have unlimited comments but still have access to important information when listing posts without having to go back to the database.

I hope this helps