fastapi__fastapi/docs/en/docs/tutorial/sql-databases.md

# SQL (Relational) Databases

**FastAPI** doesn't require you to use a SQL (relational) database.

But you can use any relational database that you want.

Here we'll see an example using <a href="https://www.sqlalchemy.org/" class="external-link" target="_blank">SQLAlchemy</a>.

You can easily adapt it to any database supported by SQLAlchemy, like:

* PostgreSQL
* MySQL
* SQLite
* Oracle
* Microsoft SQL Server, etc.

In this example, we'll use **SQLite**, because it uses a single file and Python has integrated support. So, you can copy this example and run it as is.

Later, for your production application, you might want to use a database server like **PostgreSQL**.

!!! tip
    There is an official project generator with **FastAPI** and **PostgreSQL**, all based on **Docker**, including a frontend and more tools: <a href="https://github.com/tiangolo/full-stack-fastapi-postgresql" class="external-link" target="_blank">https://github.com/tiangolo/full-stack-fastapi-postgresql</a>

!!! note
    Notice that most of the code is the standard `SQLAlchemy` code you would use with any framework.

    The **FastAPI** specific code is as small as always.

## ORMs

**FastAPI** works with any database and any style of library to talk to the database.

A common pattern is to use an "ORM": an "object-relational mapping" library.

An ORM has tools to convert ("*map*") between *objects* in code and database tables ("*relations*").

With an ORM, you normally create a class that represents a table in a SQL database, each attribute of the class represents a column, with a name and a type.

For example a class `Pet` could represent a SQL table `pets`.

And each *instance* object of that class represents a row in the database.

For example an object `orion_cat` (an instance of `Pet`) could have an attribute `orion_cat.type`, for the column `type`. And the value of that attribute could be, e.g. `"cat"`.

These ORMs also have tools to make the connections or relations between tables or entities.

This way, you could also have an attribute `orion_cat.owner` and the owner would contain the data for this pet's owner, taken from the table *owners*.

So, `orion_cat.owner.name` could be the name (from the `name` column in the `owners` table) of this pet's owner.

It could have a value like `"Arquilian"`.

And the ORM will do all the work to get the information from the corresponding table *owners* when you try to access it from your pet object.

Common ORMs are for example: Django-ORM (part of the Django framework), SQLAlchemy ORM (part of SQLAlchemy, independent of framework) and Peewee (independent of framework), among others.

Here we will see how to work with **SQLAlchemy ORM**.

In a similar way you could use any other ORM.

!!! tip
    There's an equivalent article using Peewee here in the docs.

## File structure

For these examples, let's say you have a directory named `my_super_project` that contains a sub-directory called `sql_app` with a structure like this:

```
.
└── sql_app
    ├── __init__.py
    ├── crud.py
    ├── database.py
    ├── main.py
    ├── models.py
    └── schemas.py
```

The file `__init__.py` is just an empty file, but it tells Python that `sql_app` with all its modules (Python files) is a package.

Now let's see what each file/module does.

## Create the SQLAlchemy parts

Let's refer to the file `sql_app/database.py`.

### Import the SQLAlchemy parts

```Python hl_lines="1-3"
{!../../../docs_src/sql_databases/sql_app/database.py!}
```

### Create a database URL for SQLAlchemy

```Python hl_lines="5-6"
{!../../../docs_src/sql_databases/sql_app/database.py!}
```

In this example, we are "connecting" to a SQLite database (opening a file with the SQLite database).

The file will be located at the same directory in the file `sql_app.db`.

That's why the last part is `./sql_app.db`.

If you were using a **PostgreSQL** database instead, you would just have to uncomment the line:

```Python
SQLALCHEMY_DATABASE_URL = "postgresql://user:password@postgresserver/db"
```

...and adapt it with your database data and credentials (equivalently for MySQL, MariaDB or any other).

!!! tip

    This is the main line that you would have to modify if you wanted to use a different database.

### Create the SQLAlchemy `engine`

The first step is to create a SQLAlchemy "engine".

We will later use this `engine` in other places.

```Python hl_lines="8-10"
{!../../../docs_src/sql_databases/sql_app/database.py!}
```

#### Note

The argument:

```Python
connect_args={"check_same_thread": False}
```

...is needed only for `SQLite`. It's not needed for other databases.

!!! info "Technical Details"

    By default SQLite will only allow one thread to communicate with it, assuming that each thread would handle an independent request.

    This is to prevent accidentally sharing the same connection for different things (for different requests).

    But in FastAPI, using normal functions (`def`) more than one thread could interact with the database for the same request, so we need to make SQLite know that it should allow that with `connect_args={"check_same_thread": False}`.

    Also, we will make sure each request gets its own database connection session in a dependency, so there's no need for that default mechanism.

### 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.

We will use `Session` (the one imported from SQLAlchemy) later.

To create the `SessionLocal` class, use the function `sessionmaker`:

```Python hl_lines="11"
{!../../../docs_src/sql_databases/sql_app/database.py!}
```

### Create a `Base` class

Now we will use the function `declarative_base()` that returns a class.

Later we will inherit from this class to create each of the database models or classes (the ORM models):

```Python hl_lines="13"
{!../../../docs_src/sql_databases/sql_app/database.py!}
```

## Create the database models

Let's now see the file `sql_app/models.py`.

### Create SQLAlchemy models from the `Base` class

We will use this `Base` class we created before to create the SQLAlchemy models.

!!! tip
    SQLAlchemy uses the term "**model**" to refer to these classes and instances that interact with the database.

    But Pydantic also uses the term "**model**" to refer to something different, the data validation, conversion, and documentation classes and instances.

Import `Base` from `database` (the file `database.py` from above).

Create classes that inherit from it.

These classes are the SQLAlchemy models.

```Python hl_lines="4  7-8  18-19"
{!../../../docs_src/sql_databases/sql_app/models.py!}
```

The `__tablename__` attribute tells SQLAlchemy the name of the table to use in the database for each of these models.

### Create model attributes/columns

Now create all the model (class) attributes.

Each of these attributes represents a column in its corresponding database table.

We use `Column` from SQLAlchemy as the default value.

And we pass a SQLAlchemy class "type", as `Integer`, `String`, and `Boolean`, that defines the type in the database, as an argument.

```Python hl_lines="1  10-13  21-24"
{!../../../docs_src/sql_databases/sql_app/models.py!}
```

### Create the relationships

Now create the relationships.

For this, we use `relationship` provided by SQLAlchemy ORM.

This will become, more or less, a "magic" attribute that will contain the values from other tables related to this one.

```Python hl_lines="2  15  26"
{!../../../docs_src/sql_databases/sql_app/models.py!}
```

When accessing the attribute `items` in a `User`, as in `my_user.items`, it will have a list of `Item` SQLAlchemy models (from the `items` table) that have a foreign key pointing to this record in the `users` table.

When you access `my_user.items`, SQLAlchemy will actually go and fetch the items from the database in the `items` table and populate them here.

And when accessing the attribute `owner` in an `Item`, it will contain a `User` SQLAlchemy model from the `users` table. It will use the `owner_id` attribute/column with its foreign key to know which record to get from the `users` table.

## Create the Pydantic models

Now let's check the file `sql_app/schemas.py`.

!!! tip
    To avoid confusion between the SQLAlchemy *models* and the Pydantic *models*, we will have the file `models.py` with the SQLAlchemy models, and the file `schemas.py` with the Pydantic models.

    These Pydantic models define more or less a "schema" (a valid data shape).

    So this will help us avoiding confusion while using both.

### Create initial Pydantic *models* / schemas

Create an `ItemBase` and `UserBase` Pydantic *models* (or let's say "schemas") to have common attributes while creating or reading data.

And create an `ItemCreate` and `UserCreate` that inherit from them (so they will have the same attributes), plus any additional data (attributes) needed for creation.

So, the user will also have a `password` when creating it.

But for security, the `password` won't be in other Pydantic *models*, for example, it won't be sent from the API when reading a user.

=== "Python 3.6 and above"

    ```Python hl_lines="3  6-8  11-12  23-24  27-28"
    {!> ../../../docs_src/sql_databases/sql_app/schemas.py!}
    ```

=== "Python 3.9 and above"

    ```Python hl_lines="3  6-8  11-12  23-24  27-28"
    {!> ../../../docs_src/sql_databases/sql_app_py39/schemas.py!}
    ```

=== "Python 3.10 and above"

    ```Python hl_lines="1  4-6  9-10  21-22  25-26"
    {!> ../../../docs_src/sql_databases/sql_app_py310/schemas.py!}
    ```

#### SQLAlchemy style and Pydantic style

Notice that SQLAlchemy *models* define attributes using `=`, and pass the type as a parameter to `Column`, like in:

```Python
name = Column(String)
```

while Pydantic *models* declare the types using `:`, the new type annotation syntax/type hints:

```Python
name: str
```

Have it in mind, so you don't get confused when using `=` and `:` with them.

### Create Pydantic *models* / schemas for reading / returning

Now create Pydantic *models* (schemas) that will be used when reading data, when returning it from the API.

For example, before creating an item, we don't know what will be the ID assigned to it, but when reading it (when returning it from the API) we will already know its ID.

The same way, when reading a user, we can now declare that `items` will contain the items that belong to this user.

Not only the IDs of those items, but all the data that we defined in the Pydantic *model* for reading items: `Item`.

=== "Python 3.6 and above"

    ```Python hl_lines="15-17  31-34"
    {!> ../../../docs_src/sql_databases/sql_app/schemas.py!}
    ```

=== "Python 3.9 and above"

    ```Python hl_lines="15-17  31-34"
    {!> ../../../docs_src/sql_databases/sql_app_py39/schemas.py!}
    ```

=== "Python 3.10 and above"

    ```Python hl_lines="13-15  29-32"
    {!> ../../../docs_src/sql_databases/sql_app_py310/schemas.py!}
    ```

!!! tip
    Notice that the `User`, the Pydantic *model* that will be used when reading a user (returning it from the API) doesn't include the `password`.

### Use Pydantic's `orm_mode`

Now, in the Pydantic *models* for reading, `Item` and `User`, add an internal `Config` class.

This <a href="https://pydantic-docs.helpmanual.io/#config" class="external-link" target="_blank">`Config`</a> class is used to provide configurations to Pydantic.

In the `Config` class, set the attribute `orm_mode = True`.

=== "Python 3.6 and above"

    ```Python hl_lines="15  19-20  31  36-37"
    {!> ../../../docs_src/sql_databases/sql_app/schemas.py!}
    ```

=== "Python 3.9 and above"

    ```Python hl_lines="15  19-20  31  36-37"
    {!> ../../../docs_src/sql_databases/sql_app_py39/schemas.py!}
    ```

=== "Python 3.10 and above"

    ```Python hl_lines="13  17-18  29  34-35"
    {!> ../../../docs_src/sql_databases/sql_app_py310/schemas.py!}
    ```

!!! tip
    Notice it's assigning a value with `=`, like:

    `orm_mode = True`

    It doesn't use `:` as for the type declarations before.

    This is setting a config value, not declaring a type.

Pydantic's `orm_mode` will tell the Pydantic *model* to read the data even if it is not a `dict`, but an ORM model (or any other arbitrary object with attributes).

This way, instead of only trying to get the `id` value from a `dict`, as in:

```Python
id = data["id"]
```

it will also try to get it from an attribute, as in:

```Python
id = data.id
```

And with this, the Pydantic *model* is compatible with ORMs, and you can just declare it in the `response_model` argument in your *path operations*.

You will be able to return a database model and it will read the data from it.

#### Technical Details about ORM mode

SQLAlchemy and many others are by default "lazy loading".

That means, for example, that they don't fetch the data for relationships from the database unless you try to access the attribute that would contain that data.

For example, accessing the attribute `items`:

```Python
current_user.items
```

would make SQLAlchemy go to the `items` table and get the items for this user, but not before.

Without `orm_mode`, if you returned a SQLAlchemy model from your *path operation*, it wouldn't include the relationship data.

Even if you declared those relationships in your Pydantic models.

But with ORM mode, as Pydantic itself will try to access the data it needs from attributes (instead of assuming a `dict`), you can declare the specific data you want to return and it will be able to go and get it, even from ORMs.

## CRUD utils

Now let's see the file `sql_app/crud.py`.

In this file we will have reusable functions to interact with the data in the database.

**CRUD** comes from: **C**reate, **R**ead, **U**pdate, and **D**elete.

...although in this example we are only creating and reading.

### Read data

Import `Session` from `sqlalchemy.orm`, this will allow you to declare the type of the `db` parameters and have better type checks and completion in your functions.

Import `models` (the SQLAlchemy models) and `schemas` (the Pydantic *models* / schemas).

Create utility functions to:

* Read a single user by ID and by email.
* Read multiple users.
* Read multiple items.

```Python hl_lines="1  3  6-7  10-11  14-15  27-28"
{!../../../docs_src/sql_databases/sql_app/crud.py!}
```

!!! tip
    By creating functions that are only dedicated to interacting with the database (get a user or an item) independent of your *path operation function*, you can more easily reuse them in multiple parts and also add <abbr title="Automated tests, written in code, that check if another piece of code is working correctly.">unit tests</abbr> for them.

### Create data

Now create utility functions to create data.

The steps are:

* Create a SQLAlchemy model *instance* with your data.
* `add` that instance object to your database session.
* `commit` the changes to the database (so that they are saved).
* `refresh` your instance (so that it contains any new data from the database, like the generated ID).

```Python hl_lines="18-24  31-36"
{!../../../docs_src/sql_databases/sql_app/crud.py!}
```

!!! tip
    The SQLAlchemy model for `User` contains a `hashed_password` that should contain a secure hashed version of the password.

    But as what the API client provides is the original password, you need to extract it and generate the hashed password in your application.

    And then pass the `hashed_password` argument with the value to save.

!!! warning
    This example is not secure, the password is not hashed.

    In a real life application you would need to hash the password and never save them in plaintext.

    For more details, go back to the Security section in the tutorial.

    Here we are focusing only on the tools and mechanics of databases.

!!! tip
    Instead of passing each of the keyword arguments to `Item` and reading each one of them from the Pydantic *model*, we are generating a `dict` with the Pydantic *model*'s data with:

    `item.dict()`

    and then we are passing the `dict`'s key-value pairs as the keyword arguments to the SQLAlchemy `Item`, with:

    `Item(**item.dict())`

    And then we pass the extra keyword argument `owner_id` that is not provided by the Pydantic *model*, with:

    `Item(**item.dict(), owner_id=user_id)`

## Main **FastAPI** app

And now in the file `sql_app/main.py` let's integrate and use all the other parts we created before.

### Create the database tables

In a very simplistic way create the database tables:

=== "Python 3.6 and above"

    ```Python hl_lines="9"
    {!> ../../../docs_src/sql_databases/sql_app/main.py!}
    ```

=== "Python 3.9 and above"

    ```Python hl_lines="7"
    {!> ../../../docs_src/sql_databases/sql_app_py39/main.py!}
    ```

#### Alembic Note

Normally you would probably initialize your database (create tables, etc) with <a href="https://alembic.sqlalchemy.org/en/latest/" class="external-link" target="_blank">Alembic</a>.

And you would also use Alembic for "migrations" (that's its main job).

A "migration" is the set of steps needed whenever you change the structure of your SQLAlchemy models, add a new attribute, etc. to replicate those changes in the database, add a new column, a new table, etc.

You can find an example of Alembic in a FastAPI project in the templates from [Project Generation - Template](../project-generation.md){.internal-link target=_blank}. Specifically in <a href="https://github.com/tiangolo/full-stack-fastapi-postgresql/tree/master/%7B%7Bcookiecutter.project_slug%7D%7D/backend/app/alembic/" class="external-link" target="_blank">the `alembic` directory in the source code</a>.

### Create a dependency

Now use the `SessionLocal` class we created in the `sql_app/databases.py` file to create a dependency.

We need to have an independent database session/connection (`SessionLocal`) per request, use the same session through all the request and then close it after the request is finished.

And then a new session will be created for the next request.

For that, we will create a new dependency with `yield`, as explained before in the section about [Dependencies with `yield`](dependencies/dependencies-with-yield.md){.internal-link target=_blank}.

Our dependency will create a new SQLAlchemy `SessionLocal` that will be used in a single request, and then close it once the request is finished.

=== "Python 3.6 and above"

    ```Python hl_lines="15-20"
    {!> ../../../docs_src/sql_databases/sql_app/main.py!}
    ```

=== "Python 3.9 and above"

    ```Python hl_lines="13-18"
    {!> ../../../docs_src/sql_databases/sql_app_py39/main.py!}
    ```

!!! info
    We put the creation of the `SessionLocal()` and handling of the requests in a `try` block.

    And then we close it in the `finally` block.

    This way we make sure the database session is always closed after the request. Even if there was an exception while processing the request.

    But you can't raise another exception from the exit code (after `yield`). See more in [Dependencies with `yield` and `HTTPException`](./dependencies/dependencies-with-yield.md#dependencies-with-yield-and-httpexception){.internal-link target=_blank}

And then, when using the dependency in a *path operation function*, we declare it with the type `Session` we imported directly from SQLAlchemy.

This will then give us better editor support inside the *path operation function*, because the editor will know that the `db` parameter is of type `Session`:

=== "Python 3.6 and above"

    ```Python hl_lines="24  32  38  47  53"
    {!> ../../../docs_src/sql_databases/sql_app/main.py!}
    ```

=== "Python 3.9 and above"

    ```Python hl_lines="22  30  36  45  51"
    {!> ../../../docs_src/sql_databases/sql_app_py39/main.py!}
    ```

!!! info "Technical Details"
    The parameter `db` is actually of type `SessionLocal`, but this class (created with `sessionmaker()`) is a "proxy" of a SQLAlchemy `Session`, so, the editor doesn't really know what methods are provided.

    But by declaring the type as `Session`, the editor now can know the available methods (`.add()`, `.query()`, `.commit()`, etc) and can provide better support (like completion). The type declaration doesn't affect the actual object.

### Create your **FastAPI** *path operations*

Now, finally, here's the standard **FastAPI** *path operations* code.

=== "Python 3.6 and above"

    ```Python hl_lines="23-28  31-34  37-42  45-49  52-55"
    {!> ../../../docs_src/sql_databases/sql_app/main.py!}
    ```

=== "Python 3.9 and above"

    ```Python hl_lines="21-26  29-32  35-40  43-47  50-53"
    {!> ../../../docs_src/sql_databases/sql_app_py39/main.py!}
    ```

We are creating the database session before each request in the dependency with `yield`, and then closing it afterwards.

And then we can create the required dependency in the *path operation function*, to get that session directly.

With that, we can just call `crud.get_user` directly from inside of the *path operation function* and use that session.

!!! tip
    Notice that the values you return are SQLAlchemy models, or lists of SQLAlchemy models.

    But as all the *path operations* have a `response_model` with Pydantic *models* / schemas using `orm_mode`, the data declared in your Pydantic models will be extracted from them and returned to the client, with all the normal filtering and validation.

!!! tip
    Also notice that there are `response_models` that have standard Python types like `List[schemas.Item]`.

    But as the content/parameter of that `List` is a Pydantic *model* with `orm_mode`, the data will be retrieved and returned to the client as normally, without problems.

### About `def` vs `async def`

Here we are using SQLAlchemy code inside of the *path operation function* and in the dependency, and, in turn, it will go and communicate with an external database.

That could potentially require some "waiting".

But as SQLAlchemy doesn't have compatibility for using `await` directly, as would be with something like:

```Python
user = await db.query(User).first()
```

...and instead we are using:

```Python
user = db.query(User).first()
```

Then we should declare the *path operation functions* and the dependency without `async def`, just with a normal `def`, as:

```Python hl_lines="2"
@app.get("/users/{user_id}", response_model=schemas.User)
def read_user(user_id: int, db: Session = Depends(get_db)):
    db_user = crud.get_user(db, user_id=user_id)
    ...
```

!!! info
    If you need to connect to your relational database asynchronously, see [Async SQL (Relational) Databases](../advanced/async-sql-databases.md){.internal-link target=_blank}.

!!! note "Very Technical Details"
    If you are curious and have a deep technical knowledge, you can check the very technical details of how this `async def` vs `def` is handled in the [Async](../async.md#very-technical-details){.internal-link target=_blank} docs.

## Migrations

Because we are using SQLAlchemy directly and we don't require any kind of plug-in for it to work with **FastAPI**, we could integrate database <abbr title="Automatically updating the database to have any new column we define in our models.">migrations</abbr> with <a href="https://alembic.sqlalchemy.org" class="external-link" target="_blank">Alembic</a> directly.

And as the code related to SQLAlchemy and the SQLAlchemy models lives in separate independent files, you would even be able to perform the migrations with Alembic without having to install FastAPI, Pydantic, or anything else.

The same way, you would be able to use the same SQLAlchemy models and utilities in other parts of your code that are not related to **FastAPI**.

For example, in a background task worker with <a href="https://docs.celeryproject.org" class="external-link" target="_blank">Celery</a>, <a href="https://python-rq.org/" class="external-link" target="_blank">RQ</a>, or <a href="https://arq-docs.helpmanual.io/" class="external-link" target="_blank">ARQ</a>.

## Review all the files

 Remember you should have a directory named `my_super_project` that contains a sub-directory called `sql_app`.

`sql_app` should have the following files:

* `sql_app/__init__.py`: is an empty file.

* `sql_app/database.py`:

```Python
{!../../../docs_src/sql_databases/sql_app/database.py!}
```

* `sql_app/models.py`:

```Python
{!../../../docs_src/sql_databases/sql_app/models.py!}
```

* `sql_app/schemas.py`:

=== "Python 3.6 and above"

    ```Python
    {!> ../../../docs_src/sql_databases/sql_app/schemas.py!}
    ```

=== "Python 3.9 and above"

    ```Python
    {!> ../../../docs_src/sql_databases/sql_app_py39/schemas.py!}
    ```

=== "Python 3.10 and above"

    ```Python
    {!> ../../../docs_src/sql_databases/sql_app_py310/schemas.py!}
    ```

* `sql_app/crud.py`:

```Python
{!../../../docs_src/sql_databases/sql_app/crud.py!}
```

* `sql_app/main.py`:

=== "Python 3.6 and above"

    ```Python
    {!> ../../../docs_src/sql_databases/sql_app/main.py!}
    ```

=== "Python 3.9 and above"

    ```Python
    {!> ../../../docs_src/sql_databases/sql_app_py39/main.py!}
    ```

## Check it

You can copy this code and use it as is.

!!! info

    In fact, the code shown here is part of the tests. As most of the code in these docs.

Then you can run it with Uvicorn:


<div class="termy">

```console
$ uvicorn sql_app.main:app --reload

<span style="color: green;">INFO</span>:     Uvicorn running on http://127.0.0.1:8000 (Press CTRL+C to quit)
```

</div>

And then, you can open your browser at <a href="http://127.0.0.1:8000/docs" class="external-link" target="_blank">http://127.0.0.1:8000/docs</a>.

And you will be able to interact with your **FastAPI** application, reading data from a real database:

<img src="/img/tutorial/sql-databases/image01.png">

## Interact with the database directly

If you want to explore the SQLite database (file) directly, independently of FastAPI, to debug its contents, add tables, columns, records, modify data, etc. you can use <a href="https://sqlitebrowser.org/" class="external-link" target="_blank">DB Browser for SQLite</a>.

It will look like this:

<img src="/img/tutorial/sql-databases/image02.png">

You can also use an online SQLite browser like <a href="https://inloop.github.io/sqlite-viewer/" class="external-link" target="_blank">SQLite Viewer</a> or <a href="https://extendsclass.com/sqlite-browser.html" class="external-link" target="_blank">ExtendsClass</a>.

## Alternative DB session with middleware

If you can't use dependencies with `yield` -- for example, if you are not using **Python 3.7** and can't install the "backports" mentioned above for **Python 3.6** -- you can set up the session in a "middleware" in a similar way.

A "middleware" is basically a function that is always executed for each request, with some code executed before, and some code executed after the endpoint function.

### Create a middleware

The middleware we'll add (just a function) will create a new SQLAlchemy `SessionLocal` for each request, add it to the request and then close it once the request is finished.

=== "Python 3.6 and above"

    ```Python hl_lines="14-22"
    {!> ../../../docs_src/sql_databases/sql_app/alt_main.py!}
    ```

=== "Python 3.9 and above"

    ```Python hl_lines="12-20"
    {!> ../../../docs_src/sql_databases/sql_app_py39/alt_main.py!}
    ```

!!! info
    We put the creation of the `SessionLocal()` and handling of the requests in a `try` block.

    And then we close it in the `finally` block.

    This way we make sure the database session is always closed after the request. Even if there was an exception while processing the request.

### About `request.state`

`request.state` is a property of each `Request` object. It is there to store arbitrary objects attached to the request itself, like the database session in this case. You can read more about it in <a href="https://www.starlette.io/requests/#other-state" class="external-link" target="_blank">Starlette's docs about `Request` state</a>.

For us in this case, it helps us ensure a single database session is used through all the request, and then closed afterwards (in the middleware).

### Dependencies with `yield` or middleware

Adding a **middleware** here is similar to what a dependency with `yield` does, with some differences:

* It requires more code and is a bit more complex.
* The middleware has to be an `async` function.
    * If there is code in it that has to "wait" for the network, it could "block" your application there and degrade performance a bit.
    * Although it's probably not very problematic here with the way `SQLAlchemy` works.
    * But if you added more code to the middleware that had a lot of <abbr title="input and output">I/O</abbr> waiting, it could then be problematic.
* A middleware is run for *every* request.
    * So, a connection will be created for every request.
    * Even when the *path operation* that handles that request didn't need the DB.

!!! tip
    It's probably better to use dependencies with `yield` when they are enough for the use case.

!!! info
    Dependencies with `yield` were added recently to **FastAPI**.

    A previous version of this tutorial only had the examples with a middleware and there are probably several applications using the middleware for database session management.