# [SQLAlchemy 2.x Testing](https://docs.sqlalchemy.org/en/20/orm/session_transaction.html#joining-a-session-into-an-external-transaction-such-as-for-test-suites)

Best practices for testing SQLAlchemy ORM code (sync and async) with pytest.

## Agent Quick Path
Use this path first; read deeper sections only when needed.

1. Create engine once per test session.
2. Open connection + outer transaction per test function.
3. Bind `Session`/`AsyncSession` to that connection with `join_transaction_mode="create_savepoint"`.
4. Let code under test call `commit()` safely; rollback outer transaction after test.
5. Inject session into FastAPI via dependency override and always clear overrides.

Branching logic:
- Sync stack: use `create_engine` + `Session` fixtures.
- Async stack: use `create_async_engine` + `AsyncSession` fixtures + `pytest.mark.anyio`.
- SQLite in-memory with threaded client: use `StaticPool` when required by framework threading behavior.
- Async relationship access fails (`MissingGreenlet`): eager load (`selectinload`) or explicit refresh.

---

## Core Concepts

| Concept | Preferred approach |
|---|---|
| Isolate tests from production DB | Use an in-memory SQLite engine per test |
| Prevent data leaking between tests | Roll back at the connection level after each test |
| Inject test DB into FastAPI | Override the `get_db` / `get_session` dependency |
| Create schema for tests | Call `Base.metadata.create_all(engine)` once per session |
| Avoid lazy-load errors in async | Use `expire_on_commit=False`; use `selectinload()` for relationships |

---

## Sync SQLAlchemy + FastAPI (Recommended Pattern)

The canonical 2.0 pattern joins a test `Session` into an external transaction on a shared `Connection`, then rolls back after each test. This means `session.commit()` calls within the code under test are "committed" to a savepoint, not the real transaction, and are fully undone after the test.

```python
# tests/conftest.py
import pytest
from sqlalchemy import create_engine
from sqlalchemy.orm import Session
from fastapi.testclient import TestClient

from myapp.main import app
from myapp.db.session import get_db
from myapp.db.models import Base

@pytest.fixture(scope="session")
def engine():
    e = create_engine("sqlite://", connect_args={"check_same_thread": False})
    Base.metadata.create_all(bind=e)
    yield e
    Base.metadata.drop_all(bind=e)

@pytest.fixture()
def db_session(engine):
    connection = engine.connect()
    transaction = connection.begin()
    session = Session(bind=connection, join_transaction_mode="create_savepoint")
    yield session
    session.close()
    transaction.rollback()
    connection.close()

@pytest.fixture()
def client(db_session):
    app.dependency_overrides[get_db] = lambda: db_session
    yield TestClient(app)
    app.dependency_overrides = {}
```

Key points:
- `join_transaction_mode="create_savepoint"` means each `session.commit()` inside code under test issues a SAVEPOINT release, not a real COMMIT — everything is rolled back when the test ends.
- `scope="session"` engine + `scope="function"` session/connection gives fast table creation with full per-test isolation.
- SQLite in-memory (`sqlite://`) is preferred: no files, no cleanup, fast.

---

## Async SQLAlchemy + FastAPI

For `AsyncSession` / `AsyncEngine`, the setup mirrors the sync version but uses async fixtures and `pytest-anyio`.

```python
# tests/conftest.py
import pytest
from sqlalchemy.ext.asyncio import AsyncSession, create_async_engine, async_sessionmaker
from fastapi.testclient import TestClient  # sync client still works
from httpx import AsyncClient, ASGITransport

from myapp.main import app
from myapp.db.session import get_db
from myapp.db.models import Base

@pytest.fixture(scope="session")
def anyio_backend():
    return "asyncio"

@pytest.fixture(scope="session")
async def async_engine():
    e = create_async_engine("sqlite+aiosqlite://", connect_args={"check_same_thread": False})
    async with e.begin() as conn:
        await conn.run_sync(Base.metadata.create_all)
    yield e
    async with e.begin() as conn:
        await conn.run_sync(Base.metadata.drop_all)
    await e.dispose()

@pytest.fixture()
async def async_session(async_engine):
    async with async_engine.connect() as conn:
        await conn.begin()
        session = AsyncSession(bind=conn, join_transaction_mode="create_savepoint",
                               expire_on_commit=False)
        yield session
        await session.close()
        await conn.rollback()

@pytest.fixture()
async def async_client(async_session):
    async def override_get_db():
        yield async_session

    app.dependency_overrides[get_db] = override_get_db
    async with AsyncClient(transport=ASGITransport(app=app), base_url="http://test") as ac:
        yield ac
    app.dependency_overrides = {}
```

Notes:
- Async fixtures need `@pytest.mark.anyio` on the test or `anyio_backend` session fixture.
- `expire_on_commit=False` prevents expired-attribute access on objects after `await session.commit()`.
- Install: `aiosqlite`, `anyio[asyncio]`, `httpx`.

---

## SQLModel Pattern (FastAPI + SQLModel)

SQLModel's official test pattern uses `StaticPool` + in-memory SQLite, with separate `session` and `client` pytest fixtures.

```python
import pytest
from fastapi.testclient import TestClient
from sqlmodel import Session, SQLModel, create_engine
from sqlmodel.pool import StaticPool

from myapp.main import app, get_session  # get_session is the SQLModel dependency

@pytest.fixture(name="session")
def session_fixture():
    engine = create_engine(
        "sqlite://",
        connect_args={"check_same_thread": False},
        poolclass=StaticPool,
    )
    SQLModel.metadata.create_all(engine)
    with Session(engine) as session:
        yield session

@pytest.fixture(name="client")
def client_fixture(session: Session):
    app.dependency_overrides[get_session] = lambda: session
    yield TestClient(app)
    app.dependency_overrides.clear()

# --- Tests ---

def test_create_hero(client: TestClient):
    response = client.post("/heroes/", json={"name": "Deadpond", "secret_name": "Dive Wilson"})
    assert response.status_code == 200

def test_read_heroes(session: Session, client: TestClient):
    # Directly insert test data via the session — no HTTP call needed for setup
    from myapp.models import Hero
    session.add(Hero(name="Test Hero", secret_name="Hidden"))
    session.commit()
    response = client.get("/heroes/")
    assert response.status_code == 200
    assert len(response.json()) == 1
```

Rules:
- Use `StaticPool` so a single in-memory SQLite connection is shared across threads (required by `TestClient`'s threading model).
- Both the `client` fixture and test functions can receive the same `session` — insert data directly for controlled setup rather than via the API.
- Always call `app.dependency_overrides.clear()` in fixture teardown (after `yield`).

---

## Async Session: Avoiding Implicit I/O

SQLAlchemy async has strict rules about lazy loading — attributes that would trigger IO on access will raise an error.

```python
# WRONG — will raise MissingGreenlet / lazy load error
result = await session.execute(select(User))
user = result.scalars().one()
print(user.posts)  # lazy load, fails in async context

# RIGHT — use selectinload() to load relationships eagerly
from sqlalchemy.orm import selectinload

result = await session.execute(
    select(User).options(selectinload(User.posts))
)
user = result.scalars().one()
print(user.posts)  # already loaded, no IO needed
```

Other strategies:
- `AsyncAttrs` mixin: access any attribute as an awaitable via `await obj.awaitable_attrs.relationship_name`.
- `write_only` relationships: never loaded implicitly; queried explicitly.
- `await session.refresh(obj, ["attribute_name"])`: force-load a specific attribute after the fact.

---

## Fixture Scope Decision Table

| What to scope | `scope` | Reason |
|---|---|---|
| Engine + DDL (`create_all`) | `session` | Expensive; shared across all tests |
| Connection + Transaction | `function` | Rolled back per test for isolation |
| Session | `function` | One transaction per test |
| TestClient / AsyncClient | `function` | Depends on session; recreated per test |

---

## Common Pitfalls

| Problem | Cause | Fix |
|---|---|---|
| `MissingGreenlet` in async | Lazy-loaded relationship accessed outside awaitable context | Use `selectinload()` or `AsyncAttrs.awaitable_attrs` |
| `RuntimeError: Event loop is closed` | `AsyncEngine` not disposed | Call `await engine.dispose()` in fixture teardown |
| Tests share state / data bleeds | Session not rolled back | Use `join_transaction_mode="create_savepoint"` + rollback pattern |
| `StaticPool` not used with SQLite in-memory | TestClient spawns threads that get separate in-memory DBs | Always add `poolclass=StaticPool` for in-memory SQLite |
| `expire_on_commit=True` (default) breaks async | Accessing attributes after commit triggers lazy IO | Set `expire_on_commit=False` on AsyncSession |
| Not resetting `dependency_overrides` | Override persists into next test | Always clear in fixture teardown, after `yield` |

---

## Official Docs

- [Joining a Session into an External Transaction (test suites)](https://docs.sqlalchemy.org/en/20/orm/session_transaction.html#joining-a-session-into-an-external-transaction-such-as-for-test-suites)
- [Asynchronous I/O (asyncio)](https://docs.sqlalchemy.org/en/20/orm/extensions/asyncio.html)
- [Preventing Implicit IO when Using AsyncSession](https://docs.sqlalchemy.org/en/20/orm/extensions/asyncio.html#preventing-implicit-io-when-using-asyncsession)
- [SQLModel: Test Applications with FastAPI](https://sqlmodel.tiangolo.com/tutorial/fastapi/tests/)
- [FastAPI: Testing Dependencies with Overrides](https://fastapi.tiangolo.com/advanced/testing-dependencies/)