Question

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, declarative model class, and scoped session automatically, so you can skip those parts of the SQLAlchemy tutorial.

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

Configure the Extension

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.

from flask import Flask
from flask_sqlalchemy import SQLAlchemy

# create the extension
db = SQLAlchemy()
# 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)

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

See Configuration for an explanation of connections strings and what other configuration keys are used. The SQLAlchemy object also takes some arguments to customize the objects it manages.

Define Models

Subclass db.Model to define a model class. The db object makes the names in sqlalchemy and sqlalchemy.orm available for convenience, such as db.Column. The model will generate a table name by converting the CamelCase class name to snake_case.

class User(db.Model):
    id = db.Column(db.Integer, primary_key=True)
    username = db.Column(db.String, unique=True, nullable=False)
    email = db.Column(db.String)

The table name "user" will automatically be assigned to the model’s table.

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.