How do I decide whether a post is liked by a user with Mongoose

I am trying to make a function so that users can like posts (either a post is liked by the user or it is not liked by the user, i.e. no dislike or voting).

My posts are objects in a MongoDB collection with schema

{
  title: String,
  text: String
}

while my users have schema

{
  username: String,
  password: String
}

I could, for instance, create an array at posts with the ids of the users that have liked the specific post. It would look something like

{
  title: String,
  text: String,
  likedByUsers: [ObjectID]
}

or I could create an array at users with the ids of the posts that the user has liked, which would be something like

{
  username: String,
  password: String,
  postsLiked: [ObjectID]
}

However, I expect that the users will like thousands of posts, so I might face a restriction in the size of the objects in MongoDB.

So I am thinking that I should instead create a new collection with likes

{
  userId: ObjectID,
  postId: ObjectID
}

However, I am thinking how I should retrieve it in my app.

Should I store an array of the IDs of all the posts that the user (who is logged in) has liked, and when I am printing each post, I will look if the post is among the posts that the user has liked, and then show an "already liked" button?

Alternatively, I could send the user id when I am requesting to see all the posts, and then for each post add a field saying "isLiked", representing whether an object in the Liked collection could be find, matching both the post and the user id?