Add conftest.py for globally used fixtures

backend/src/routes/conftest.py

@@ -0,0 +1,39 @@
+# Conftest.py includes globally used functions. These are especially fixtures, which are used for dependency injection.
+# See https://www.tutorialspoint.com/pytest/pytest_conftest_py.htm
+# See https://gist.github.com/peterhurford/09f7dcda0ab04b95c026c60fa49c2a68
+
+import pytest
+from sqlmodel import create_engine, Session, SQLModel
+from fastapi.testclient import TestClient
+from sqlmodel.pool import StaticPool
+
+from src import database
+from src.main import app
+
+
+@pytest.fixture(name="session")
+def session_fixture():
+    """Creates a mock session, that access an in-memory temporary database."""
+
+    engine = create_engine(
+        "sqlite://",  # In memory database
+        connect_args={"check_same_thread": False},  # FastAPI's async functions may execute on different threads
+        poolclass=StaticPool  # All threads should access shared memory
+    )
+    SQLModel.metadata.create_all(engine)
+    with Session(engine) as session:
+        yield session
+
+
+@pytest.fixture(name="client")
+def client_fixture(session: Session):
+    """Creates a FastAPI TestClient with a mocked session dependency. This causes FastAPI to inject the fake session
+    into path operation functions, so that we have full control over the mocked database."""
+
+    def get_session_override():
+        return session
+
+    app.dependency_overrides[database.get_session] = get_session_override
+    client = TestClient(app)
+    yield client
+    app.dependency_overrides.clear()