Mainly, there are 2 approaches to achieve that goal.
You can embed the replies as a field in the post document. Something like:
post.insert
(
[
comments:
[
{title:"Spelling wrong",
content:"You spelled attenttion wrong…",
likes:1009,
replies: []}
]
]
);
Or, using another collection for replies that holds the key-value of the appropriate comment (like in relational databases).
For more information you can see the MongoDB documentation about: Data Modeling.