Challenge 1: Write the Post.comments Resolver — Possible Solution
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const resolvers = {
  // ...existing Query, User, Post.author resolvers from the chapter...
  Post: {
    author: (parent) => db.users.findById(parent.authorId),
    comments: (parent) => db.comments.findByPostId(parent.id), // parent = the Post
  },
};

WHY THIS WORKS
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- comments is added as a SIBLING to the existing author resolver inside
  the Post section of the resolver map — both are resolvers for fields
  defined on the Post type in the schema, so they belong in the same
  object.
- parent here is whatever Post object was returned by whichever resolver
  ran one level up (e.g. Query.posts, or User.posts) — exactly the same
  rule the chapter walked through for Post.author's parent.authorId
  lookup. Since parent IS the specific Post being resolved, parent.id is
  that post's own ID — the correct value to pass into
  db.comments.findByPostId().
- This deliberately mirrors the shape of the existing Post.author
  resolver in the chapter's example — both take parent (the Post) and
  use one of its own properties (authorId for author, id for comments)
  to look up related data through a different top-level collection
  (db.users vs db.comments) — the same "look up related data via
  parent's own field" pattern repeated for a new relationship.
- No args parameter is needed here (only parent), because comments
  isn't queried with any arguments of its own in this schema — the
  chapter's Query.user resolver needed args specifically because id was
  passed as an argument to that particular field; Post.comments doesn't
  take any arguments, so there's nothing there to read.