transaction-example.ts

@Post('money/transfer')
async transferMoney(@Body() dto: TransferMoneyDto) {
  // Start session using mongo connection
  const session = await this.mongoConnection.startSession();
  // Start transaction using the session
  session.startTransaction();

  try {
    // This method will do a few mongo db operations
    const result = await this.transferMoneyService.transfer(dto, session);

    // If transferMoneyService.transfer is resolved, we then commit the transaction
    // This will do the actual writes/updates to the database
    await session.commitTransaction();

    return result;
  } catch (e) {
    // If the error is thrown, we will abort the transaction, so nothing will be changed in the database
    await session.abortTransaction();
    throw e
  } finally {
    // Make sure to end transaction session.
    session.endSession();
  }
}


// The method that will update 2 Mongo documents
async transfer(dto: TransferMoneyDto, session: ClientSession) {
  const { fromAccountId, transferAmount, toAccountId } = dto;
  // Make sure to put session in the query options
  await this.accountModel.findByIdAndUpdate(fromAccountId, { $inc: { amount: -1 * transferAmount } }, { session }).exec()
  await this.accountModel.findByIdAndUpdate(toAccountId, { $inc: { amount: +1 * transferAmount }}, { session }).exec()
}