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
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.
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
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.
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.
db.Model to define a model class.
The model will generate a table name by converting the
CamelCase class name to
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
method to get a list of results, or the
Result.scalar() method to get a single
@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
See Modifying and Querying Data for more information about queries.
What to Remember¶
For the most part, you should use SQLAlchemy as usual. The
instance creates, configures, and gives access to the following things:
SQLAlchemy.Modeldeclarative model base class. It sets the table name automatically instead of needing
SQLAlchemy.sessionis a session that is scoped to the current Flask application context. It is cleaned up after every request.
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.