Testing a Database

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You can use the same dependency overrides from to alter a database for testing.

You could want to set up a different database for testing, rollback the data after the tests, pre-fill it with some testing data, etc.

The main idea is exactly the same you saw in that previous chapter.

Let’s update the example from to use a testing database.

All the app code is the same, you can go back to that chapter check how it was.

The only changes here are in the new testing file.

In the test, you could use a dependency override to return your custom database session instead of the one that would be used normally.

In this example we’ll create a temporary database only for the tests.

File structure

We create a new file at sql_app/tests/test_sql_app.py.

So the new file structure looks like:

First, we create a new database session with the new database.

For the tests we’ll use a file test.db instead of sql_app.db.

But the rest of the session code is more or less the same, we just copy it.

from fastapi.testclient import TestClient
from sqlalchemy import create_engine
from sqlalchemy.orm import sessionmaker
from ..database import Base
from ..main import app, get_db
SQLALCHEMY_DATABASE_URL = "sqlite:///./test.db"
engine = create_engine(
    SQLALCHEMY_DATABASE_URL, connect_args={"check_same_thread": False}
)
TestingSessionLocal = sessionmaker(autocommit=False, autoflush=False, bind=engine)
Base.metadata.create_all(bind=engine)
def override_get_db():
    try:
        db = TestingSessionLocal()
        yield db
    finally:
        db.close()
app.dependency_overrides[get_db] = override_get_db
client = TestClient(app)
def test_create_user():
    response = client.post(
        "/users/",
        json={"email": "deadpool@example.com", "password": "chimichangas4life"},
    )
    assert response.status_code == 200, response.text
    data = response.json()
    assert data["email"] == "deadpool@example.com"
    assert "id" in data
    user_id = data["id"]
    assert response.status_code == 200, response.text
    data = response.json()
    assert data["email"] == "deadpool@example.com"
    assert data["id"] == user_id

Tip

For simplicity and to focus on the specific testing code, we are just copying it.

Create the database

Because now we are going to use a new database in a new file, we need to make sure we create the database with:

That is normally called in main.py, but the line in main.py uses the database file sql_app.db, and we need to make sure we create test.db for the tests.

So we add that line here, with the new file.

from fastapi.testclient import TestClient
from sqlalchemy import create_engine
from sqlalchemy.orm import sessionmaker
from ..database import Base
from ..main import app, get_db
SQLALCHEMY_DATABASE_URL = "sqlite:///./test.db"
engine = create_engine(
    SQLALCHEMY_DATABASE_URL, connect_args={"check_same_thread": False}
)
TestingSessionLocal = sessionmaker(autocommit=False, autoflush=False, bind=engine)
Base.metadata.create_all(bind=engine)
def override_get_db():
    try:
        db = TestingSessionLocal()
        yield db
    finally:
        db.close()
app.dependency_overrides[get_db] = override_get_db
client = TestClient(app)
def test_create_user():
    response = client.post(
        "/users/",
        json={"email": "deadpool@example.com", "password": "chimichangas4life"},
    )
    assert response.status_code == 200, response.text
    data = response.json()
    assert data["email"] == "deadpool@example.com"
    assert "id" in data
    user_id = data["id"]
    response = client.get(f"/users/{user_id}")
    assert response.status_code == 200, response.text
    assert data["email"] == "deadpool@example.com"
    assert data["id"] == user_id

Now we create the dependency override and add it to the overrides for our app.

Tip

The code for is almost exactly the same as for get_db(), but in override_get_db() we use the TestingSessionLocal for the testing database instead.

Test the app

Then we can just test the app as normally.

from fastapi.testclient import TestClient
from sqlalchemy import create_engine
from sqlalchemy.orm import sessionmaker
from ..database import Base
from ..main import app, get_db
SQLALCHEMY_DATABASE_URL = "sqlite:///./test.db"
engine = create_engine(
    SQLALCHEMY_DATABASE_URL, connect_args={"check_same_thread": False}
)
TestingSessionLocal = sessionmaker(autocommit=False, autoflush=False, bind=engine)
Base.metadata.create_all(bind=engine)
def override_get_db():
    try:
        db = TestingSessionLocal()
        yield db
    finally:
        db.close()
app.dependency_overrides[get_db] = override_get_db
client = TestClient(app)
def test_create_user():
    response = client.post(
        "/users/",
        json={"email": "deadpool@example.com", "password": "chimichangas4life"},
    )
    assert response.status_code == 200, response.text
    data = response.json()
    assert data["email"] == "deadpool@example.com"
    assert "id" in data
    user_id = data["id"]
    response = client.get(f"/users/{user_id}")
    assert response.status_code == 200, response.text
    data = response.json()
    assert data["email"] == "deadpool@example.com"