Exercise 2: Merging Two Tags — Possible Solution
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async function mergeTags(fromName: string, intoName: string) {
  if (fromName === intoName) throw new Error("Can't merge a tag into itself");

  return prisma.$transaction(async (tx) => {
    const from = await tx.tag.findUnique({
      where:   { name: fromName },
      include: { posts: { select: { id: true } } },
    });
    const into = await tx.tag.findUnique({ where: { name: intoName } });
    if (!from || !into) throw new Error("Both tags must exist");

    // Add the target tag to every post that had the old one.
    // connect is a no-op for posts that already have it.
    for (const post of from.posts) {
      await tx.post.update({
        where: { id: post.id },
        data:  { tags: { connect: { id: into.id } } },
      });
    }

    // Deleting the tag removes its links from the implicit join table
    await tx.tag.delete({ where: { id: from.id } });

    return from.posts.length;
  });
}

// Usage
const moved = await mergeTags("js", "javascript");
console.log(`Moved ${moved} posts`);

WHY THIS WORKS AS AN ANSWER
------------------------------
The later steps depend on what the first queries find (which posts
have the old tag, and whether both tags exist), so this needs an
interactive transaction rather than the batch form. Every query uses
tx, not prisma. Throwing when a tag is missing rolls back anything
already done, so a half-finished merge is impossible. The loop is fine
for a small blog; for thousands of posts you'd use raw SQL on the join
table instead (Chapter 3) to keep the transaction short.