Test database strategies in NestJS - Step-by-Step Execution

Concept Flow - Test database strategies

Start Test Setup

↓

Choose Test DB Strategy

↓

In-Memory

↓

Initialize DB

↓

Run Tests Using DB

↓

Clean Up DB

↓

End Test

This flow shows how tests start by choosing a database strategy, then initialize or mock the database, run tests, clean up, and finish.

Execution Sample

NestJS

beforeAll(async () => {
  await testContainer.start();
  await app.init();
});

afterAll(async () => {
  await testContainer.stop();
});

This code starts a test database container before tests and stops it after all tests finish.

Execution Table

Step	Action	Evaluation	Result
1	Start test setup	N/A	Test environment initialized
2	Choose test DB strategy	Using Test Container	Docker container for DB started
3	Initialize DB connection	Connect to container DB	DB connection established
4	Run tests	Execute queries and mutations	Tests run against container DB
5	Clean up DB	Drop test data	DB cleaned for next test
6	Stop test container	Shutdown container	Container stopped
7	End test	N/A	Test suite finished

💡 Test container stopped and test suite finished

Variable Tracker

Variable	Start	After Step 2	After Step 3	After Step 4	After Step 5	Final
testContainer	undefined	started	started	started	started	stopped
dbConnection	undefined	undefined	connected	connected	connected	closed
testData	empty	empty	empty	populated	cleared	empty

Key Moments - 3 Insights

Why do we need to clean up the database after each test?

Cleaning up ensures tests do not affect each other by leaving data behind, as shown in step 5 of the execution_table.

What happens if the test container is not stopped after tests?

The container keeps running, wasting resources and possibly causing conflicts, as indicated in step 6 where the container is stopped.

Why choose a test container over a real database?

Test containers provide isolated, fresh environments for tests, avoiding side effects on real databases, as shown in step 2 choosing the test container strategy.

Visual Quiz - 3 Questions

Look at the execution_table, what is the state of testContainer after Step 3?

A undefined

B started

C stopped

D paused

💡 Hint

Check variable_tracker row for testContainer after Step 3

At which step does the database get cleaned?

A Step 5

B Step 3

C Step 4

D Step 6

💡 Hint

Look at the execution_table action column for cleaning

If we skip stopping the test container, what is the likely final state of testContainer?

A undefined

B stopped

C started

D cleaned

💡 Hint

Refer to variable_tracker final state for testContainer and step 6 in execution_table

Concept Snapshot

Test database strategies in NestJS:
- Choose strategy: In-memory, Test Container, Mock, or Real DB
- Initialize DB before tests
- Run tests using DB connection
- Clean DB after each test
- Stop/cleanup resources after all tests
Use test containers for isolated, fresh DB environments.

Full Transcript

This visual execution shows how to manage test database strategies in NestJS. First, the test setup starts and selects a database strategy like a test container. The container starts, and the app connects to it. Tests run using this database. After tests, the database is cleaned to remove test data. Finally, the test container is stopped to free resources. Variables like testContainer and dbConnection change states accordingly. Cleaning the database prevents tests from interfering with each other. Stopping the container avoids resource waste. Choosing the right strategy helps keep tests reliable and isolated.