Personal Catalogue: React, Express & MongoDB — Chapter 5, Exercise 1
=====================================================================

TASK
Build both PATCH routes (add and remove) and the TagEditor component,
then add and remove several tags on a real Book, confirming after each
step via GET /api/items/:id that duplicates are never created and
removed tags are genuinely gone.

SOLUTION

routes/items.js (added after Chapter 3's existing routes):

  router.patch('/:id/tags/add', async (req, res) => {
    const { tag } = req.body;
    if (!tag) return res.status(400).json({ error: 'tag is required' });
    const updated = await Book.findByIdAndUpdate(
      req.params.id,
      { $addToSet: { tags: tag } },
      { new: true, runValidators: true }
    ).lean();
    if (!updated) return res.status(404).json({ error: 'Item not found' });
    res.json(updated);
  });

  router.patch('/:id/tags/remove', async (req, res) => {
    const { tag } = req.body;
    if (!tag) return res.status(400).json({ error: 'tag is required' });
    const updated = await Book.findByIdAndUpdate(
      req.params.id,
      { $pull: { tags: tag } },
      { new: true }
    ).lean();
    if (!updated) return res.status(404).json({ error: 'Item not found' });
    res.json(updated);
  });

Testing sequence, starting from a real Book with tags: ["Python"]:

  curl -X PATCH http://localhost:4000/api/items/<id>/tags/add \
    -H "Content-Type: application/json" -d '{"tag":"Programming"}'
  curl http://localhost:4000/api/items/<id>
  # → tags: ["Python", "Programming"]

  curl -X PATCH http://localhost:4000/api/items/<id>/tags/add \
    -H "Content-Type: application/json" -d '{"tag":"Python"}'
  curl http://localhost:4000/api/items/<id>
  # → tags: ["Python", "Programming"] — unchanged, no duplicate added

  curl -X PATCH http://localhost:4000/api/items/<id>/tags/remove \
    -H "Content-Type: application/json" -d '{"tag":"Python"}'
  curl http://localhost:4000/api/items/<id>
  # → tags: ["Programming"] — Python is genuinely gone, Programming untouched

  curl -X PATCH http://localhost:4000/api/items/<id>/tags/remove \
    -H "Content-Type: application/json" -d '{"tag":"Nonexistent"}'
  curl http://localhost:4000/api/items/<id>
  # → tags: ["Programming"] — removing a tag that was never there is a
  #   harmless no-op, not an error

Run with: npx nodemon server.js, then the curl sequence above
Output: at each checkpoint, the GET confirms the tags array holds
exactly what the sequence of add/remove operations implies -- no
duplicates ever appear regardless of how many times the same tag is
added, and a removed tag never reappears on a later, unrelated read.

WHY THIS WORKS AS AN ANSWER
----------------------------
It builds both routes exactly as the chapter specifies, then verifies
their real behavior with an actual sequence of operations against a
live database rather than trusting the code to behave correctly --
confirming both the no-duplicate guarantee $addToSet provides and the
genuine removal $pull performs, plus the harmless no-op case for
removing a tag that isn't present.