Challenge 2: Append to an Array — Possible Solution
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db.products.updateOne(
  { name: "Notebook" },
  { $push: { tags: "sale" } }
)

WHY THIS WORKS AS AN ANSWER
------------------------------
$push is specifically the operator for adding a new element onto the
END of an existing array field, without disturbing whatever elements
are already there — exactly what "add a new tag" calls for. Using
$set here instead would be wrong: $set: { tags: "sale" } would REPLACE
the entire tags array with the single string "sale", destroying any
tags that were already on the product, rather than adding to them.
$push is the operator that specifically means "append," which is the
one this scenario actually needs.