Transactions in NestJS - Mini Project: Build & Apply

Handling Transactions in NestJS

📖 Scenario: You are building a simple NestJS service to manage bank accounts. You want to ensure that money transfers between accounts happen safely using transactions, so that either both accounts update correctly or none do.

🎯 Goal: Build a NestJS service method that uses transactions to transfer money between two accounts safely.

📋 What You'll Learn

Create an initial accounts data structure with two accounts and their balances

Add a variable to hold the transfer amount

Write a service method that uses a transaction to update both accounts

Complete the transaction handling with commit and rollback logic

💡 Why This Matters

🌍 Real World

Transactions ensure that money transfers between accounts happen safely without data loss or corruption.

💼 Career

Understanding transactions is essential for backend developers working with databases and NestJS to build reliable financial or data-critical applications.

1

Create initial accounts data

Create a variable called accounts as a JavaScript object with two keys: accountA with value 1000 and accountB with value 500.

NestJS

// Create the accounts object with balances
// Your code here

Hint

Use a JavaScript object with keys accountA and accountB and assign the given balances.

2

Add transfer amount variable

Add a constant variable called transferAmount and set it to 200.

NestJS

const accounts = { accountA: 1000, accountB: 500 };
// Define the transfer amount
// Your code here

Hint

Use const transferAmount = 200; to define the amount to transfer.

3

Write transfer method with transaction

Write an async function called transferMoney that accepts no parameters. Inside, use a try block to simulate starting a transaction by logging 'Transaction started'. Then subtract transferAmount from accounts.accountA and add it to accounts.accountB. After that, log 'Transaction committed'. Add a catch block that logs 'Transaction rolled back' if an error occurs.

NestJS

const accounts = { accountA: 1000, accountB: 500 };
const transferAmount = 200;

// Write the transferMoney async function
// Your code here

Hint

Use try and catch blocks inside an async function to simulate transaction behavior.

4

Complete transaction with NestJS style

Modify the transferMoney function to accept a queryRunner parameter. Use await queryRunner.startTransaction() at the start, then update the accounts. After updating, use await queryRunner.commitTransaction(). In the catch block, use await queryRunner.rollbackTransaction(). Finally, use await queryRunner.release() in a finally block.

NestJS

const accounts = { accountA: 1000, accountB: 500 };
const transferAmount = 200;

// Modify transferMoney to use queryRunner for transaction
// Your code here

Hint

Use await with queryRunner methods to handle the transaction lifecycle properly.

Practice

1. What is the main purpose of using transactions in a NestJS application with a database?

easy

A. To automatically generate database schemas

B. To ensure multiple database operations succeed or fail together

C. To speed up database queries by caching results

D. To log all database queries for debugging

Solution

1. Step 1: Understand what transactions do

   Transactions group multiple database actions so they either all succeed or all fail, preventing partial updates.

2. Step 2: Identify the purpose in NestJS context

   In NestJS, transactions keep data consistent by ensuring grouped operations are atomic.

3. Final Answer:

   To ensure multiple database operations succeed or fail together -> Option B

4. Quick Check:

   Transactions = atomic operations [OK]

Hint: Transactions keep grouped DB actions all-or-nothing [OK]

Common Mistakes:

• Thinking transactions speed up queries
• Confusing transactions with schema generation
• Assuming transactions are for logging only

2. Which of the following is the correct way to start a transaction using TypeORM's DataSource in NestJS?

easy

A. dataSource.transaction(async (manager) => { /* operations */ })

B. dataSource.startTransaction(() => { /* operations */ })

C. dataSource.begin(async () => { /* operations */ })

D. dataSource.runTransaction(() => { /* operations */ })

Solution

1. Step 1: Recall TypeORM transaction method

   TypeORM uses dataSource.transaction with an async callback to run transactional code.

2. Step 2: Check method names

   Only dataSource.transaction(async (manager) => { /* operations */ }) uses the correct method name and async callback pattern.

3. Final Answer:

   dataSource.transaction(async (manager) => { /* operations */ }) -> Option A

4. Quick Check:

   Correct method is transaction with async callback [OK]

Hint: Use dataSource.transaction with async callback [OK]

Common Mistakes:

• Using non-existent methods like startTransaction
• Omitting async keyword in callback
• Using wrong method names like begin or runTransaction

3. Given the following code snippet in a NestJS service using TypeORM, what will be the output if the second operation fails?

await dataSource.transaction(async (manager) => {
  await manager.save(user1);
  await manager.save(user2); // throws error
});
console.log('Transaction complete');

medium

A. Neither user1 nor user2 are saved, and an error is thrown before logging

B. Only user1 is saved, then 'Transaction complete' is logged

C. Both user1 and user2 are saved, then 'Transaction complete' is logged

D. 'Transaction complete' is logged before any save operations

Solution

1. Step 1: Understand transaction rollback behavior

   If any operation inside the transaction fails, all changes are rolled back, so no data is saved.

2. Step 2: Analyze the code flow

   The second save throws an error, so the transaction aborts and neither user1 nor user2 are saved. The error prevents reaching the console.log.

3. Final Answer:

   Neither user1 nor user2 are saved, and an error is thrown before logging -> Option A

4. Quick Check:

   Transaction rollback on error = no saves, error thrown [OK]

Hint: Error inside transaction rolls back all changes [OK]

Common Mistakes:

• Assuming partial saves happen before error
• Thinking console.log runs despite error
• Believing transaction commits despite failure

4. Identify the error in this NestJS transaction code snippet:

await dataSource.transaction(async (manager) => {
  manager.save(user1);
  await manager.save(user2);
});

medium

A. dataSource.transaction requires a synchronous callback

B. Transaction callback should not be async

C. Missing await before manager.save(user1)

D. manager.save cannot be used inside transactions

Solution

1. Step 1: Check async calls inside transaction

   All asynchronous operations inside the transaction callback must be awaited to ensure proper execution order.

2. Step 2: Identify missing await

   The first save call lacks await, so it may not complete before the second save starts, causing unexpected behavior.

3. Final Answer:

   Missing await before manager.save(user1) -> Option C

4. Quick Check:

   Await all async DB calls inside transaction [OK]

Hint: Always await async DB calls inside transaction [OK]

Common Mistakes:

• Omitting await on async calls
• Thinking transaction callback can't be async
• Believing manager.save is disallowed in transactions

5. You want to update two related entities atomically in NestJS using TypeORM transactions. Which approach correctly ensures both updates succeed or fail together?

async updateEntities(entityA, entityB) {
  await this.dataSource.transaction(async (manager) => {
    await manager.save(entityA);
    await manager.save(entityB);
  });
}

What is the best practice to handle errors and maintain data consistency?

hard

A. Manually call rollback() inside the transaction callback on error

B. Save entities without transactions and rely on database constraints

C. Use separate transactions for each save to isolate failures

D. Wrap the transaction call in try-catch and handle rollback automatically

Solution

1. Step 1: Understand transaction error handling in TypeORM

   TypeORM automatically rolls back if an error is thrown inside the transaction callback.

2. Step 2: Use try-catch outside transaction to handle errors

   Wrapping the transaction call in try-catch lets you catch errors and respond without manual rollback calls.

3. Final Answer:

   Wrap the transaction call in try-catch and handle rollback automatically -> Option D

4. Quick Check:

   Try-catch outside transaction handles errors; rollback is automatic [OK]

Hint: Use try-catch around transaction; rollback is automatic [OK]

Common Mistakes:

• Trying to manually rollback inside transaction callback
• Using separate transactions for related updates
• Ignoring transactions and relying only on DB constraints