from . import crud, models, schemas
from .database import SessionLocal, engine

models.Base.metadata.create_all(bind=engine)

app = FastAPI()


@app.middleware("http")
async def db_session_middleware(request: Request, call_next):
    response = Response("Internal server error", status_code=500)
    try:
        request.state.db = SessionLocal()
        response = await call_next(request)
    finally:

    此外，我们将确保每个请求都在依赖项中获得自己的数据库连接会话，因此不需要该默认机制。

### 创建一个`SessionLocal`类

每个实例`SessionLocal`都会是一个数据库会话。当然该类本身还不是数据库会话。

但是一旦我们创建了一个`SessionLocal`类的实例，这个实例将是实际的数据库会话。

我们命名它是`SessionLocal`为了将它与我们从 SQLAlchemy 导入的`Session`区别开来。

稍后我们将使用`Session`（从 SQLAlchemy 导入的那个）。

要创建`SessionLocal`类，请使用函数`sessionmaker`：

```Python hl_lines="11"

from fastapi import Depends, FastAPI, HTTPException
from sqlalchemy.orm import Session

from . import crud, models, schemas
from .database import SessionLocal, engine

models.Base.metadata.create_all(bind=engine)

app = FastAPI()


# Dependency
def get_db():
    db = SessionLocal()
    try:
        yield db
    finally:
        db.close()


@app.post("/users/", response_model=schemas.User)

    , 👥 🔜 ⚒ 💭 🔠 📨 🤚 🚮 👍 💽 🔗 🎉 🔗, 📤 🙅‍♂ 💪 👈 🔢 🛠️.

### ✍ `SessionLocal` 🎓

🔠 👐 `SessionLocal` 🎓 🔜 💽 🎉. 🎓 ⚫️ 🚫 💽 🎉.

✋️ 🕐 👥 ✍ 👐 `SessionLocal` 🎓, 👉 👐 🔜 ☑ 💽 🎉.

👥 📛 ⚫️ `SessionLocal` 🔬 ⚫️ ⚪️➡️ `Session` 👥 🏭 ⚪️➡️ 🇸🇲.

👥 🔜 ⚙️ `Session` (1️⃣ 🗄 ⚪️➡️ 🇸🇲) ⏪.

✍ `SessionLocal` 🎓, ⚙️ 🔢 `sessionmaker`:

```Python hl_lines="11"

SQLALCHEMY_DATABASE_URL = "sqlite:///./sql_app.db"
# SQLALCHEMY_DATABASE_URL = "postgresql://user:password@postgresserver/db"

engine = create_engine(
    SQLALCHEMY_DATABASE_URL, connect_args={"check_same_thread": False}
)
SessionLocal = sessionmaker(autocommit=False, autoflush=False, bind=engine)

from fastapi import Depends, FastAPI, HTTPException
from sqlalchemy.orm import Session

from . import crud, models, schemas
from .database import SessionLocal, engine

models.Base.metadata.create_all(bind=engine)

app = FastAPI()


# Dependency
def get_db():
    db = SessionLocal()
    try:
        yield db
    finally:
        db.close()


@app.post("/users/", response_model=schemas.User)

### Create a `SessionLocal` class

Each instance of the `SessionLocal` class will be a database session. The class itself is not a database session yet.

But once we create an instance of the `SessionLocal` class, this instance will be the actual database session.

We name it `SessionLocal` to distinguish it from the `Session` we are importing from SQLAlchemy.

from fastapi import Depends, FastAPI, HTTPException
from sqlalchemy.orm import Session

from . import crud, models, schemas
from .database import SessionLocal, engine

models.Base.metadata.create_all(bind=engine)

app = FastAPI()


# Dependency
def get_db():
    db = SessionLocal()
    try:
        yield db
    finally:
        db.close()


@app.post("/users/", response_model=schemas.User)

from . import crud, models, schemas
from .database import SessionLocal, engine

models.Base.metadata.create_all(bind=engine)

app = FastAPI()


@app.middleware("http")
async def db_session_middleware(request: Request, call_next):
    response = Response("Internal server error", status_code=500)
    try:
        request.state.db = SessionLocal()
        response = await call_next(request)
    finally:

from . import crud, models, schemas
from .database import SessionLocal, engine

models.Base.metadata.create_all(bind=engine)

app = FastAPI()


@app.middleware("http")
async def db_session_middleware(request: Request, call_next):
    response = Response("Internal server error", status_code=500)
    try:
        request.state.db = SessionLocal()
        response = await call_next(request)
    finally: