المعاملات (Transactions)

const session = await mongoose.startSession();
session.startTransaction();

try {
  await Account.updateOne(
    { _id: from }, { $inc: { balance: -100 } }, { session }
  );
  await Account.updateOne(
    { _id: to }, { $inc: { balance: 100 } }, { session }
  );

  await session.commitTransaction();   // نجاح: ثبّت الكل
} catch (err) {
  await session.abortTransaction();    // فشل: تراجع عن الكل
} finally {
  session.endSession();
}