How do you handle Mongoose transactions for multi-document atomic operations in NestJS?

Q: How do you handle Mongoose transactions for multi-document atomic operations in NestJS?

@Injectable() export class OrdersService { constructor( @InjectConnection() private readonly connection: Connection, @InjectModel(Order.name) private orderModel: Model , @InjectModel(Product.name) private productModel: Model , ) {} async placeOrder(dto: CreateOrderDto): Promise { const session = await this.connection.startSession(); session.startTransaction(); try { const [order] = await this.orderModel.create( [{ ...dto }], { session }, // pass session to every operation ); await this.productModel.findByIdAndUpdate( dto.productId, { $inc: { stock: -dto.qty } }, { session, new: true }, ); await session.commitTransaction(); return order; } catch (error) { await session.abortTransaction(); throw error; } finally { session.endSession(); // always release the session } } } Mongoose transactions require a MongoDB replica set — single-node standalone instances do not support them. Use @InjectConnection() to get the Mongoose Connection for creating sessions. Pass { session } to every query inside the transaction or the operation runs outside it. Model.create() with a session requires the array form: create([data], { session }). Always call session.endSession() in finally — not releasing it leaks the session from the pool.

Difficulty: 6/10

Mongoose sessions, NestJS providers, error handling

Inject the Mongoose Connection with @InjectConnection() and call startSession() to create a session. Start a transaction on the session and pass { session } to every query that must participate. Commit on success, abort on error, and always end the session in a finally block.

Mongoose multi-document transaction

Mongoose transaction requirements and rules:

1
Mongoose transactions require a MongoDB replica set — single-node standalone instances do not support them.
2
Use @InjectConnection() to get the Mongoose Connection for creating sessions.
3
Pass { session } to every query inside the transaction or the operation runs outside it.
4
Model.create() with a session requires the array form: create([data], { session }).
5
Always call session.endSession() in finally — not releasing it leaks the session from the pool.

Scenario Questions

0-2 years experience

2-5 years experience

1You added a transaction to a checkout flow, but the request sometimes hangs and never returns. Walk me through how you'd debug the problem.

How do you handle Mongoose transactions for multi-document atomic operations in NestJS?

Inject the Mongoose Connection with @InjectConnection() and call startSession() to create a session. Start a transaction on the session and pass { session } to every query that must participate. Commit on success, abort on error, and always end the session in a finally block.

Scenario Questions

Follow-up Questions