Challenge 1: Write an updatePost Mutation — Possible Solution
====================================================================

// Schema

input UpdatePostInput {
  id: ID!
  title: String
}

type Mutation {
  createPost(input: CreatePostInput!): Post!
  updatePost(input: UpdatePostInput!): Post!
}

// Resolver

Mutation: {
  createPost: (parent, args, context) => {
    return db.posts.create({
      title: args.input.title,
      authorId: args.input.authorId,
    });
  },
  updatePost: (parent, args, context) => {
    return db.posts.update(args.input.id, {
      title: args.input.title,
    });
  },
}

WHY THIS WORKS
--------------
- UpdatePostInput requires id: ID! (non-null) because there is no
  reasonable way to update a post without knowing WHICH post to update
  — but title: String is left nullable/optional, since a real update
  operation typically only needs to change SOME fields, not require the
  client to resend every field on every update. This mirrors a common,
  practical pattern: required identifier, optional fields to change.
- updatePost follows the exact same structural pattern as createPost
  from the chapter — an input type wrapping the arguments, a Mutation
  field taking that input type as its sole argument, and a resolver
  reading args.input.* to perform the actual write — the only real
  difference is which underlying database operation it calls (update
  vs create) and which fields the input type actually needs.
- Returning Post! (not just a Boolean) follows the chapter's "return the
  updated object" convention directly — a client calling updatePost can
  immediately query the post's current title (or any other Post field)
  in the same round trip, exactly as the chapter's createPost example
  demonstrated for a newly created post.
- db.posts.update(args.input.id, { title: args.input.title }) uses
  args.input.id specifically to identify WHICH record to modify — this
  is the id the UPDATE operation needs, which is exactly why it was
  made a required (non-null) field on the input type, unlike title.