Challenge 3: Write an Upsert — Possible Solution
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db.visitCounters.updateOne(
  { userId: "u42" },
  { $inc: { visits: 1 } },
  { upsert: true }
)

WHY THIS WORKS AS AN ANSWER
------------------------------
This is exactly the pattern the chapter's page-view example
demonstrated, applied to a per-user counter instead of a per-page one:

  - The FIRST time this runs for userId "u42" (no matching document
    exists yet), the upsert: true option causes MongoDB to CREATE a
    new document instead of doing nothing. Because $inc on a
    non-existent field treats the starting value as 0, the created
    document ends up as { userId: "u42", visits: 1 } — exactly what
    the challenge asked for.
  - EVERY subsequent time this runs for the same userId, a matching
    document already exists, so $inc simply increments its existing
    visits field by 1 instead of creating anything new.

The key insight being tested: upsert: true doesn't require writing
separate "insert if missing" and "update if present" logic yourself —
the single updateOne call with upsert: true handles both cases
automatically, based purely on whether the filter finds a match.