Test database strategies in NestJS - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ component_behavior

intermediate

What happens when using an in-memory database for NestJS tests?

You configure your NestJS tests to use an in-memory database like SQLite in memory mode. What is the main behavior you will observe during test runs?

A Data from previous test runs remains in the database, causing tests to share state.

B Each test run starts with a fresh empty database, so no data persists between tests.

C The database runs on disk and requires manual cleanup after tests.

D The in-memory database automatically syncs with the production database.

💡 Hint

Think about what 'in-memory' means for data persistence.

In-memory databases store data only during the test run in RAM. When the test ends, all data is lost, so each test run starts fresh.

📝 Syntax

intermediate

Which NestJS testing setup correctly resets the test database before each test?

Given the following NestJS test setup, which option correctly resets the database before each test?

NestJS

beforeEach(async () => {
  // reset database here
});

A await connection.synchronize(true);

B await connection.clear();

C await connection.dropDatabase(); await connection.synchronize();

D await connection.close();

💡 Hint

Resetting means removing all tables and recreating them.

Calling dropDatabase() removes all tables, and synchronize() recreates them based on the entities, ensuring a clean database state before each test.

🔧 Debug

advanced

Why does the NestJS test suite fail when using a shared test database without cleanup?

You run your NestJS tests using a shared test database but do not clean data between tests. Tests start failing with unique constraint errors. What is the cause?

A The test runner does not support parallel execution.

B The database connection is closed too early causing errors.

C The test database schema is not synchronized with entities.

D Tests share data causing conflicts because previous test data remains in the database.

💡 Hint

Think about what happens if data is not cleared between tests.

Without cleaning, data from one test remains and can cause conflicts like duplicate keys in the next test.

🧠 Conceptual

advanced

What is the advantage of using transactions in NestJS test database strategies?

Why might you wrap each NestJS test in a database transaction that is rolled back after the test?

A It isolates test data changes and automatically reverts them, keeping the database clean.

B It speeds up tests by caching database queries.

C It permanently saves test data for debugging after tests finish.

D It allows tests to share data between each other for faster setup.

💡 Hint

Think about how transactions can undo changes.

Transactions let you group changes and then roll them back, so tests do not leave data behind.

❓ state_output

expert

What is the final count of users in the test database after running this NestJS test code?

Consider this NestJS test code using TypeORM with an in-memory SQLite database. What is the number of users in the database after the test completes?

NestJS

it('adds users and rolls back', async () => {
  await connection.transaction(async manager => {
    await manager.save(User, { name: 'Alice' });
    await manager.save(User, { name: 'Bob' });
    const countInside = await manager.count(User);
    expect(countInside).toBe(2);
    throw new Error('Rollback');
  }).catch(() => {});
  const countAfter = await connection.manager.count(User);
  expect(countAfter).toBe(0);
});

A 0 users, because the transaction was rolled back.

B 1 user, because only the first save succeeded.

C 2 users, because the users were saved before the error.

D Test fails with an unhandled error.

💡 Hint

What happens to data inside a transaction if an error is thrown?

The transaction rolls back all changes when an error occurs, so no users remain after the test.

Practice

1.

Why do we use a separate test database in NestJS applications?

easy

A. To store user passwords securely

B. To keep real data safe and avoid accidental changes during tests

C. To make the application run faster in production

D. To reduce the size of the main database

Solution

1. Step 1: Understand the purpose of test databases

   Test databases are used to isolate testing data from real data to prevent accidental data loss or corruption.

2. Step 2: Identify the main reason for using test databases in NestJS

   Using a separate test database ensures tests do not affect production data, keeping it safe.

3. Final Answer:

   To keep real data safe and avoid accidental changes during tests -> Option B

4. Quick Check:

   Test database = Protect real data [OK]

Hint: Test DB isolates test data from real data [OK]

Common Mistakes:

• Thinking test DB improves production speed
• Confusing test DB with security for passwords
• Assuming test DB reduces main DB size

2.

Which of the following is the correct way to configure an in-memory SQLite database for testing in a NestJS TypeOrmModule?

TypeOrmModule.forRoot({
  type: 'sqlite',
  database: ???,
  entities: [User],
  synchronize: true,
})

easy

A. 'test.db'

B. 'memory.db'

C. ':disk:'

D. ':memory:'

Solution

1. Step 1: Recall SQLite in-memory database syntax

   SQLite uses ':memory:' as the special name to create a temporary in-memory database.

2. Step 2: Match the correct database name in the config

   Using ':memory:' tells TypeORM to use an in-memory DB for fast, isolated tests.

3. Final Answer:

   ':memory:' -> Option D

4. Quick Check:

   SQLite in-memory DB = ':memory:' [OK]

Hint: SQLite in-memory DB uses ':memory:' string [OK]

Common Mistakes:

• Using a file name instead of ':memory:'
• Confusing ':disk:' as valid SQLite option
• Using arbitrary file names like 'memory.db'

3.

Given this NestJS test setup snippet, what will be the output of console.log(users.length) inside the test?

beforeEach(async () => {
  await connection.synchronize(true); // resets DB
  await userRepository.save([{ name: 'Alice' }, { name: 'Bob' }]);
});

test('users count', async () => {
  const users = await userRepository.find();
  console.log(users.length);
});

medium

A. 1

B. 0

C. 2

D. undefined

Solution

1. Step 1: Understand what synchronize(true) does

   Calling synchronize(true) resets the database, clearing all data before each test.

2. Step 2: Analyze the data saved before the test

   After reset, two users 'Alice' and 'Bob' are saved, so the DB has 2 users.

3. Step 3: Check the find() result

   Finding all users returns an array of length 2.

4. Final Answer:

   2 -> Option C

5. Quick Check:

   Reset + save 2 users = 2 users found [OK]

Hint: Reset DB then save 2 users means find() returns 2 [OK]

Common Mistakes:

• Assuming DB is not reset and counting old data
• Thinking synchronize(true) deletes data after save
• Expecting undefined because of async confusion

4.

What is the main problem with this test database setup code snippet?

beforeAll(async () => {
  await connection.synchronize(true);
});

afterAll(async () => {
  await connection.close();
});

test('should save user', async () => {
  await userRepository.save({ name: 'Charlie' });
  const users = await userRepository.find();
  expect(users.length).toBe(1);
});

test('should find no users initially', async () => {
  const users = await userRepository.find();
  expect(users.length).toBe(0);
});

medium

A. Database is reset only once before all tests, causing data to persist between tests

B. Connection is closed before tests run

C. User repository is not defined

D. Tests run synchronously causing race conditions

Solution

1. Step 1: Analyze when the database reset happens

   Database reset with synchronize(true) is done only once before all tests, not before each test.

2. Step 2: Understand test data persistence

   Data saved in the first test remains in the DB during the second test, causing unexpected results.

3. Final Answer:

   Database is reset only once before all tests, causing data to persist between tests -> Option A

4. Quick Check:

   Reset once beforeAll causes data carryover [OK]

Hint: Reset DB before each test to avoid data carryover [OK]

Common Mistakes:

• Thinking connection closes too early
• Assuming userRepository is undefined
• Believing tests run in parallel causing race

5.

You want to ensure each test in your NestJS app runs with a clean database state using an in-memory SQLite DB. Which strategy best achieves this?

1. Use TypeOrmModule.forRoot with database: ':memory:'
2. Reset the database schema before each test
3. Close and reopen the connection before each test
4. Seed only required test data inside each test

Which combination of these steps is best practice?

hard

A. All steps 1, 2, 3, and 4

B. 1 and 3 only

C. 2, 3, and 4 only

D. 1, 2, and 4 only

Solution

1. Step 1: Use in-memory DB for fast isolated tests

   Setting database to ':memory:' creates a fresh DB in memory for each connection.

2. Step 2: Reset schema before each test

   Resetting schema ensures no leftover data or schema changes affect tests.

3. Step 3: Close and reopen connection before each test

   Reopening connection resets the in-memory DB state fully.

4. Step 4: Seed only needed data per test

   Seeding minimal data keeps tests independent and fast.

5. Final Answer:

   All steps 1, 2, 3, and 4 -> Option A

6. Quick Check:

   Full reset + seed per test = reliable tests [OK]

Hint: Combine in-memory DB, reset, reconnect, and seed per test [OK]

Common Mistakes:

• Skipping connection reopen causing stale DB
• Not resetting schema before tests
• Seeding too much data globally