Quick Start — Flask-SQLAlchemy Documentation (3.1.x)

# Quick Start

Flask-SQLAlchemy simplifies using SQLAlchemy by automatically handling creating, using, and cleaning up the SQLAlchemy objects you’d normally work with. While it adds a few useful features, it still works like SQLAlchemy.

This page will walk you through the basic use of Flask-SQLAlchemy. For full capabilities and customization, see the rest of these docs, including the API docs for the `SQLAlchemy` object.

## Check the SQLAlchemy Documentation

Flask-SQLAlchemy is a wrapper around SQLAlchemy. You should follow the SQLAlchemy Tutorial to learn about how to use it, and consult its documentation for detailed information about its features. These docs show how to set up Flask-SQLAlchemy itself, not how to use SQLAlchemy. Flask-SQLAlchemy sets up the engine and scoped session automatically, so you can skip those parts of the SQLAlchemy tutorial.

This guide assumes you are using SQLAlchemy 2.x, which has a new API for defining models and better support for Python type hints and data classes. If you are using SQLAlchemy 1.x, see Legacy Quickstart.

## Installation

Flask-SQLAlchemy is available on PyPI and can be installed with various Python tools. For example, to install or update the latest version using pip:

    $ pip install -U Flask-SQLAlchemy

## Initialize the Extension

First create the `db` object using the `SQLAlchemy` constructor.

Pass a subclass of either DeclarativeBase or DeclarativeBaseNoMeta to the constructor.

    from flask import Flask
    from flask_sqlalchemy import SQLAlchemy
    from sqlalchemy.orm import DeclarativeBase

    class Base(DeclarativeBase):
      pass

    db = SQLAlchemy(model_class=Base)

Learn more about customizing the base model class in Models and Tables.

### About the `SQLAlchemy` object

Once constructed, the `db` object gives you access to the `db.Model` class to define models, and the `db.session` to execute queries.

The `SQLAlchemy` object also takes additional arguments to customize the objects it manages.

## Configure the Extension

The next step is to connect the extension to your Flask app. The only required Flask app config is the `SQLALCHEMY_DATABASE_URI` key. That is a connection string that tells SQLAlchemy what database to connect to.

Create your Flask application object, load any config, and then initialize the `SQLAlchemy` extension class with the application by calling `db.init_app`. This example connects to a SQLite database, which is stored in the app’s instance folder.

    # create the app
    app = Flask(__name__)
    # configure the SQLite database, relative to the app instance folder
    app.config["SQLALCHEMY_DATABASE_URI"] = "sqlite:///project.db"
    # initialize the app with the extension
    db.init_app(app)

See Configuration for an explanation of connections strings and what other configuration keys are used.

## Define Models

Subclass `db.Model` to define a model class. The model will generate a table name by converting the `CamelCase` class name to `snake_case`.

    from sqlalchemy import Integer, String
    from sqlalchemy.orm import Mapped, mapped_column

    class User(db.Model):
        id: Mapped[int] = mapped_column(Integer, primary_key=True)
        username: Mapped[str] = mapped_column(String, unique=True, nullable=False)
        email: Mapped[str] = mapped_column(String)

See Models and Tables for more information about defining and creating models and tables.

## Create the Tables

After all models and tables are defined, call `SQLAlchemy.create_all()` to create the table schema in the database. This requires an application context. Since you’re not in a request at this point, create one manually.

    with app.app_context():
        db.create_all()

If you define models in other modules, you must import them before calling `create_all`, otherwise SQLAlchemy will not know about them.

`create_all` does not update tables if they are already in the database. If you change a model’s columns, use a migration library like Alembic with Flask-Alembic or Flask-Migrate to generate migrations that update the database schema.

## Query the Data

Within a Flask view or CLI command, you can use `db.session` to execute queries and modify model data.

SQLAlchemy automatically defines an `__init__` method for each model that assigns any keyword arguments to corresponding database columns and other attributes.

`db.session.add(obj)` adds an object to the session, to be inserted. Modifying an object’s attributes updates the object. `db.session.delete(obj)` deletes an object. Remember to call `db.session.commit()` after modifying, adding, or deleting any data.

`db.session.execute(db.select(...))` constructs a query to select data from the database. Building queries is the main feature of SQLAlchemy, so you’ll want to read its tutorial on select to learn all about it. You’ll usually use the `Result.scalars()` method to get a list of results, or the `Result.scalar()` method to get a single result.

    @app.route("/users")
    def user_list():
        users = db.session.execute(db.select(User).order_by(User.username)).scalars()
        return render_template("user/list.html", users=users)

    @app.route("/users/create", methods=["GET", "POST"])
    def user_create():
        if request.method == "POST":
            user = User(
                username=request.form["username"],
                email=request.form["email"],
            )
            db.session.add(user)
            db.session.commit()
            return redirect(url_for("user_detail", id=user.id))

        return render_template("user/create.html")

    @app.route("/user/<int:id>")
    def user_detail(id):
        user = db.get_or_404(User, id)
        return render_template("user/detail.html", user=user)

    @app.route("/user/<int:id>/delete", methods=["GET", "POST"])
    def user_delete(id):
        user = db.get_or_404(User, id)

        if request.method == "POST":
            db.session.delete(user)
            db.session.commit()
            return redirect(url_for("user_list"))

        return render_template("user/delete.html", user=user)

You may see uses of `Model.query` to build queries. This is an older interface for queries that is considered legacy in SQLAlchemy. Prefer using `db.session.execute(db.select(...))` instead.

See Modifying and Querying Data for more information about queries.

## What to Remember

For the most part, you should use SQLAlchemy as usual. The `SQLAlchemy` extension instance creates, configures, and gives access to the following things:

- `SQLAlchemy.Model` declarative model base class. It sets the table name automatically instead of needing `__tablename__`.

- `SQLAlchemy.session` is a session that is scoped to the current Flask application context. It is cleaned up after every request.

- `SQLAlchemy.metadata` and `SQLAlchemy.metadatas` gives access to each metadata defined in the config.

- `SQLAlchemy.engine` and `SQLAlchemy.engines` gives access to each engine defined in the config.

- `SQLAlchemy.create_all()` creates all tables.

- You must be in an active Flask application context to execute queries and to access the session and engine.