How to implement a friend adding system?

Question

JavaScript

I am writing here my next facebook for the sake of studying :D and wondering how to properly implement the friend adding system.
As I understand it in my head, I have a user model, each user has a unique ID, make an additional in the user model. the field (array) in which I will push the ID of the user being added by clicking on adding to friends, as a result, an array with IDs will accumulate, if everything is clear so far, then I don’t know how to fetch the posts of all these users later.
For the first time I write a backend in general, before that I learned to typeset, if it’s not difficult, share your thoughts on how you would implement it all step by step (not that part)

Answer 1

how then to fetch the posts of all these users

db.post.find({ author_id:{ $in: friend_list }})