Test database strategies in NestJS - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to import the testing module in NestJS.

NestJS

import { Test, TestingModule } from '@nestjs/[1]';

Drag options to blanks, or click blank then click option'

Atests

Btesting

Ctest

Dtest-module

Attempts:

3 left

💡 Hint

Common Mistakes

Using '@nestjs/test' instead of '@nestjs/testing'.

Misspelling the import path as 'tests'.

The correct import path for NestJS testing utilities is '@nestjs/testing'.

2fill in blank

medium

Complete the code to create a testing module with a database provider.

NestJS

const module: TestingModule = await Test.createTestingModule({ providers: [[1]] }).compile();

Drag options to blanks, or click blank then click option'

ADbConnection

BDatabaseModule

CDatabaseProvider

DDatabaseService

Attempts:

3 left

💡 Hint

Common Mistakes

Using a module like DatabaseModule in the providers array (use imports for modules).

Using an unregistered service or connection name.

In NestJS testing, database services are typically provided by including the service class in the providers array.

3fill in blank

hard

Fix the error in the test setup by completing the database connection string.

NestJS

await module.get(DatabaseProvider).connect([1]);

Drag options to blanks, or click blank then click option'

A'mongodb://localhost:27017/testdb'

B'mysql://localhost:3306/testdb'

C'postgresql://localhost:5432/testdb'

D'redis://localhost:6379/testdb'

Attempts:

3 left

💡 Hint

Common Mistakes

Using a connection string for a different database type.

Missing quotes around the connection string.

The test database connection string should match the database type used; PostgreSQL is common for NestJS tests with TypeORM.

4fill in blank

hard

Fill both blanks to mock the database service and clear data before each test.

NestJS

jest.mock('[1]');
beforeEach(async () => { await databaseService.[2](); });

Drag options to blanks, or click blank then click option'

Asrc/database/database.service

Bclear

Csrc/database/database.provider

Dreset

Attempts:

3 left

💡 Hint

Common Mistakes

Mocking the module or provider instead of the service.

Calling a non-existent or wrong method like 'reset' without implementation.

Mock the database service file path with jest.mock and call the 'clear' method on the database service instance before each test to reset state.

5fill in blank

hard

Fill all three blanks to create a test database connection, run migrations, and close connection after tests.

NestJS

await connection.[1]();
await connection.[2]();
afterAll(async () => { await connection.[3](); });

Drag options to blanks, or click blank then click option'

Aconnect

BrunMigrations

Cclose

Dinitialize

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'initialize' instead of 'connect'.

Forgetting to close the connection in afterAll.

First connect to the test database, run migrations to set up the schema, and close the connection after all tests.

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