Exercise 3: Replacing Tags and Finding Unused Tags — Possible Solution
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// PUT /posts/:slug/tags   { "tags": ["prisma", "orm"] }
app.put("/posts/:slug/tags", async (req, res) => {
  const raw: unknown[] = Array.isArray(req.body.tags) ? req.body.tags : [];
  const names = [...new Set(raw.map((t) => String(t).trim().toLowerCase()))]
    .filter((n) => n.length > 0);

  const post = await prisma.post.update({
    where: { slug: req.params.slug },
    data: {
      tags: {
        set: [],
        connectOrCreate: names.map((name) => ({ where: { name }, create: { name } })),
      },
    },
    select: { slug: true, tags: { select: { name: true } } },
  });
  res.json(post);
});

// GET /tags/unused
app.get("/tags/unused", async (req, res) => {
  const tags = await prisma.tag.findMany({
    where:   { posts: { none: {} } },
    orderBy: { name: "asc" },
    select:  { name: true },
  });
  res.json(tags);
});

Testing with curl:

  $ curl -X PUT localhost:3000/posts/hello-api/tags \
      -H "Content-Type: application/json" -d '{"tags":["Prisma","orm","prisma"]}'
  {"slug":"hello-api","tags":[{"name":"prisma"},{"name":"orm"}]}

  $ curl localhost:3000/tags/unused
  [{"name":"api"}]           # "api" was on the post before, and now isn't

  $ curl -X PUT localhost:3000/posts/no-such-post/tags \
      -H "Content-Type: application/json" -d '{"tags":[]}'
  {"error":"Not found"}      # P2025 turned into a 404 by the error handler

WHY THIS WORKS AS AN ANSWER
------------------------------
It reuses the set + connectOrCreate pattern from Chapter 8 inside a
real route, uses the none relation filter from Chapter 9, and tests
both the success path and the error path. Note that set only removes
links — the "api" tag itself survives, which is exactly why it then
shows up as unused.