Challenge 2: Build a $facet Dashboard Query — Possible Solution
====================================================================

db.reviews.aggregate([
  { $facet: {
      averageRating: [
        { $group: { _id: null, avg: { $avg: "$rating" } } }
      ],
      recentReviews: [
        { $sort: { createdAt: -1 } },
        { $limit: 3 }
      ]
  } }
])

WHY THIS WORKS AS AN ANSWER
------------------------------
Both sub-pipelines start from the SAME input — every document in
reviews — because $facet feeds its input to each named sub-pipeline
independently, rather than passing one sub-pipeline's output into the
next (unlike every stage seen before this chapter).

averageRating uses $group with _id: null, collapsing every review
into a single result document holding one computed average — the same
"group everything into one bucket" pattern used for a grand total.

recentReviews is a completely different kind of operation — sorting
individual documents and taking the top 3 — with no grouping or
computation at all. That's the point of $facet: these two sub-pipelines
don't need to resemble each other in shape or purpose, only share the
same starting input.

The result is one document: { averageRating: [ { _id: null, avg: 4.2 } ],
recentReviews: [ {...}, {...}, {...} ] } — both answers back in a
single round trip instead of two separate queries.