Updating safely

Update operators

Product.updateOne({ _id: id }, { $set: { price: 25 } });   // set fields
Product.updateOne({ _id: id }, { $inc: { stock: -1 } });   // add or subtract
Post.updateOne({ _id: id }, { $push: { tags: "new" } });   // append to an array

updateOne resolves to a summary such as { matchedCount: 1, modifiedCount: 1 }, not the document.

Getting the document back

await Product.findByIdAndUpdate(id, { $inc: { stock: 5 } });                  // returns the OLD document
await Product.findByIdAndUpdate(id, { $inc: { stock: 5 } }, { new: true });   // returns the NEW one

This one trips everyone at least once.

The overselling race

const p = await Product.findById(id);   // A reads stock = 1   … B reads stock = 1
if (p.stock < qty) return false;        // both pass the check
p.stock -= qty; await p.save();         // both save: you sold 2 lamps you didn't have

Between your read and your write, another request can sneak in. The fix: let the database do the check and the change in one step.

const r = await Product.updateOne(
  { _id: id, stock: { $gte: qty } },    // only matches if there's enough
  { $inc: { stock: -qty } },
);
return r.modifiedCount === 1;           // 0 means someone else got there first

The exercise fires two buyers at the last lamp at the same moment. Watch the naive version fail, then fix it.