Using the serialization plugin

Our next improvement is to leverage the SQLAlchemySerializationPlugin so that we can receive and return our SQLAlchemy models directly to and from our handlers.

Here’s the code:

  1from collections.abc import AsyncGenerator
  2from contextlib import asynccontextmanager
  3from typing import Optional
  4
  5from sqlalchemy import select
  6from sqlalchemy.exc import IntegrityError, NoResultFound
  7from sqlalchemy.ext.asyncio import AsyncSession, async_sessionmaker, create_async_engine
  8from sqlalchemy.orm import DeclarativeBase, Mapped, mapped_column
  9
 10from litestar import Litestar, get, post, put
 11from litestar.contrib.sqlalchemy.plugins import SQLAlchemySerializationPlugin
 12from litestar.datastructures import State
 13from litestar.exceptions import ClientException, NotFoundException
 14from litestar.status_codes import HTTP_409_CONFLICT
 15
 16
 17class Base(DeclarativeBase): ...
 18
 19
 20class TodoItem(Base):
 21    __tablename__ = "todo_items"
 22
 23    title: Mapped[str] = mapped_column(primary_key=True)
 24    done: Mapped[bool]
 25
 26
 27@asynccontextmanager
 28async def db_connection(app: Litestar) -> AsyncGenerator[None, None]:
 29    engine = getattr(app.state, "engine", None)
 30    if engine is None:
 31        engine = create_async_engine("sqlite+aiosqlite:///todo.sqlite", echo=True)
 32        app.state.engine = engine
 33
 34    async with engine.begin() as conn:
 35        await conn.run_sync(Base.metadata.create_all)
 36
 37    try:
 38        yield
 39    finally:
 40        await engine.dispose()
 41
 42
 43sessionmaker = async_sessionmaker(expire_on_commit=False)
 44
 45
 46async def provide_transaction(state: State) -> AsyncGenerator[AsyncSession, None]:
 47    async with sessionmaker(bind=state.engine) as session:
 48        try:
 49            async with session.begin():
 50                yield session
 51        except IntegrityError as exc:
 52            raise ClientException(
 53                status_code=HTTP_409_CONFLICT,
 54                detail=str(exc),
 55            ) from exc
 56
 57
 58async def get_todo_by_title(todo_name, session: AsyncSession) -> TodoItem:
 59    query = select(TodoItem).where(TodoItem.title == todo_name)
 60    result = await session.execute(query)
 61    try:
 62        return result.scalar_one()
 63    except NoResultFound as e:
 64        raise NotFoundException(detail=f"TODO {todo_name!r} not found") from e
 65
 66
 67async def get_todo_list(done: Optional[bool], session: AsyncSession) -> list[TodoItem]:
 68    query = select(TodoItem)
 69    if done is not None:
 70        query = query.where(TodoItem.done.is_(done))
 71
 72    result = await session.execute(query)
 73    return result.scalars().all()
 74
 75
 76@get("/")
 77async def get_list(transaction: AsyncSession, done: Optional[bool] = None) -> list[TodoItem]:
 78    return await get_todo_list(done, transaction)
 79
 80
 81@post("/")
 82async def add_item(data: TodoItem, transaction: AsyncSession) -> TodoItem:
 83    transaction.add(data)
 84    return data
 85
 86
 87@put("/{item_title:str}")
 88async def update_item(item_title: str, data: TodoItem, transaction: AsyncSession) -> TodoItem:
 89    todo_item = await get_todo_by_title(item_title, transaction)
 90    todo_item.title = data.title
 91    todo_item.done = data.done
 92    return todo_item
 93
 94
 95app = Litestar(
 96    [get_list, add_item, update_item],
 97    dependencies={"transaction": provide_transaction},
 98    lifespan=[db_connection],
 99    plugins=[SQLAlchemySerializationPlugin()],
100)

 1from collections.abc import AsyncGenerator
 2from contextlib import asynccontextmanager
 3
 4from sqlalchemy import select
 5from sqlalchemy.exc import IntegrityError, NoResultFound
 6from sqlalchemy.ext.asyncio import AsyncSession, async_sessionmaker, create_async_engine
 7from sqlalchemy.orm import DeclarativeBase, Mapped, mapped_column
 8
 9from litestar import Litestar, get, post, put
10from litestar.contrib.sqlalchemy.plugins import SQLAlchemySerializationPlugin
11from litestar.datastructures import State
12from litestar.exceptions import ClientException, NotFoundException
13from litestar.status_codes import HTTP_409_CONFLICT
14
15
16class Base(DeclarativeBase): ...
17
18
19class TodoItem(Base):
20    __tablename__ = "todo_items"
21
22    title: Mapped[str] = mapped_column(primary_key=True)
23    done: Mapped[bool]
24
25
26@asynccontextmanager
27async def db_connection(app: Litestar) -> AsyncGenerator[None, None]:
28    engine = getattr(app.state, "engine", None)
29    if engine is None:
30        engine = create_async_engine("sqlite+aiosqlite:///todo.sqlite", echo=True)
31        app.state.engine = engine
32
33    async with engine.begin() as conn:
34        await conn.run_sync(Base.metadata.create_all)
35
36    try:
37        yield
38    finally:
39        await engine.dispose()
40
41
42sessionmaker = async_sessionmaker(expire_on_commit=False)
43
44
45async def provide_transaction(state: State) -> AsyncGenerator[AsyncSession, None]:
46    async with sessionmaker(bind=state.engine) as session:
47        try:
48            async with session.begin():
49                yield session
50        except IntegrityError as exc:
51            raise ClientException(
52                status_code=HTTP_409_CONFLICT,
53                detail=str(exc),
54            ) from exc
55
56
57async def get_todo_by_title(todo_name, session: AsyncSession) -> TodoItem:
58    query = select(TodoItem).where(TodoItem.title == todo_name)
59    result = await session.execute(query)
60    try:
61        return result.scalar_one()
62    except NoResultFound as e:
63        raise NotFoundException(detail=f"TODO {todo_name!r} not found") from e
64
65
66async def get_todo_list(done: bool | None, session: AsyncSession) -> list[TodoItem]:
67    query = select(TodoItem)
68    if done is not None:
69        query = query.where(TodoItem.done.is_(done))
70
71    result = await session.execute(query)
72    return result.scalars().all()
73
74
75@get("/")
76async def get_list(transaction: AsyncSession, done: bool | None = None) -> list[TodoItem]:
77    return await get_todo_list(done, transaction)
78
79
80@post("/")
81async def add_item(data: TodoItem, transaction: AsyncSession) -> TodoItem:
82    transaction.add(data)
83    return data
84
85
86@put("/{item_title:str}")
87async def update_item(item_title: str, data: TodoItem, transaction: AsyncSession) -> TodoItem:
88    todo_item = await get_todo_by_title(item_title, transaction)
89    todo_item.title = data.title
90    todo_item.done = data.done
91    return todo_item
92
93
94app = Litestar(
95    [get_list, add_item, update_item],
96    dependencies={"transaction": provide_transaction},
97    lifespan=[db_connection],
98    plugins=[SQLAlchemySerializationPlugin()],
99)

We’ve simply imported the plugin and added it to our app’s plugins list, and now we can receive and return our SQLAlchemy data models directly to and from our handler.

We’ve also been able to remove the TodoType and TodoCollectionType aliases, and the serialize_todo() function, making the implementation even more concise.

Compare handlers before and after Serialization Plugin

Once more, let’s compare the sets of application handlers before and after our refactoring:

 1@get("/")
 2async def get_list(transaction: AsyncSession, done: Optional[bool] = None) -> list[TodoItem]:
 3    return await get_todo_list(done, transaction)
 4
 5
 6@post("/")
 7async def add_item(data: TodoItem, transaction: AsyncSession) -> TodoItem:
 8    transaction.add(data)
 9    return data
10
11
12@put("/{item_title:str}")
13async def update_item(item_title: str, data: TodoItem, transaction: AsyncSession) -> TodoItem:
14    todo_item = await get_todo_by_title(item_title, transaction)
15    todo_item.title = data.title
16    todo_item.done = data.done
17    return todo_item
18
19
20app = Litestar(
21    [get_list, add_item, update_item],
22    dependencies={"transaction": provide_transaction},
23    lifespan=[db_connection],
24    plugins=[SQLAlchemySerializationPlugin()],

 1@get("/")
 2async def get_list(state: State, done: Optional[bool] = None) -> TodoCollectionType:
 3    async with sessionmaker(bind=state.engine) as session:
 4        return [serialize_todo(todo) for todo in await get_todo_list(done, session)]
 5
 6
 7@post("/")
 8async def add_item(data: TodoType, state: State) -> TodoType:
 9    new_todo = TodoItem(title=data["title"], done=data["done"])
10    async with sessionmaker(bind=state.engine) as session:
11        try:
12            async with session.begin():
13                session.add(new_todo)
14        except IntegrityError as e:
15            raise ClientException(
16                status_code=HTTP_409_CONFLICT,
17                detail=f"TODO {new_todo.title!r} already exists",
18            ) from e
19
20    return serialize_todo(new_todo)
21
22
23@put("/{item_title:str}")
24async def update_item(item_title: str, data: TodoType, state: State) -> TodoType:
25    async with sessionmaker(bind=state.engine) as session, session.begin():
26        todo_item = await get_todo_by_title(item_title, session)
27        todo_item.title = data["title"]
28        todo_item.done = data["done"]
29    return serialize_todo(todo_item)

Very nice! But, we can do better.

Next steps

In our application, we’ve had to build a bit of scaffolding to integrate SQLAlchemy with our application. We’ve had to define the db_connection() lifespan context manager, and the provide_transaction() dependency provider.

Next we’ll look at how the SQLAlchemyInitPlugin can help us.