Relations and populate

A post has an author. Don't copy the author's name into every post (it goes stale when they rename). Store a reference: the author's _id.

const postSchema = new mongoose.Schema({
  title: String,
  author: { type: mongoose.Schema.Types.ObjectId, ref: "User" },   // ref names the model
});

Reading it back

const post = await Post.findOne();
post.author;              // ObjectId("66f…"): just the id

const full = await Post.findOne().populate("author");
full.author.name;         // "Ada": the whole user document

populate runs a second lookup for you and swaps the id for the document.

Filtering by a reference

Post.find({ author: userId });   // all posts by this user

Comparing ids

post.author is an ObjectId object, not a string. post.author === req.user.id is always false. Use post.author.equals(id) or compare strings: String(post.author) === String(id).

Embed or reference?

• Embed small things that belong to one parent and are read together (an order's line items).
• Reference things that are shared or grow without limit (users, comments on a popular post).