Legacy Quickstart — Flask-SQLAlchemy Documentation (3.1.x)

# Legacy Quickstart

Warning

This guide shows you how to initialize the extension and define models when using the SQLAlchemy 1.x style of ORM model classes. We encourage you to upgrade to SQLAlchemy 2.x to take advantage of the new typed model classes.

## Initialize the Extension

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

When using the SQLAlchemy 1.x API, you do not need to pass any arguments to the `SQLAlchemy` constructor. A declarative base class will be created behind the scenes for you.

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

    db = SQLAlchemy()

## Using custom MetaData and naming conventions

You can optionally construct the `SQLAlchemy` object with a custom `MetaData` object. This allows you to specify a custom constraint naming convention. This makes constraint names consistent and predictable, useful when using migrations, as described by Alembic.

    from sqlalchemy import MetaData
    from flask_sqlalchemy import SQLAlchemy

    db = SQLAlchemy(metadata=MetaData(naming_convention={
        "ix": 'ix_%(column_0_label)s',
        "uq": "uq_%(table_name)s_%(column_0_name)s",
        "ck": "ck_%(table_name)s_%(constraint_name)s",
        "fk": "fk_%(table_name)s_%(column_0_name)s_%(referred_table_name)s",
        "pk": "pk_%(table_name)s"
    }))

## Define Models

Subclass `db.Model` to define a model class. This is a SQLAlchemy declarative base class, it will take `Column` attributes and create a table.

    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)

For convenience, the extension object provides access to names in the `sqlalchemy` and `sqlalchemy.orm` modules. So you can use `db.Column` instead of importing and using `sqlalchemy.Column`, although the two are equivalent.

Unlike plain SQLAlchemy, Flask-SQLAlchemy’s model will automatically generate a table name if `__tablename__` is not set and a primary key column is defined. The table name `"user"` will automatically be assigned to the model’s table.

## Create the Tables

Defining a model does not create it in the database. Use `create_all()` to create the models and tables after defining them. If you define models in submodules, you must import them so that SQLAlchemy knows about them before calling `create_all`.

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

## Querying the Data

You can query the data the same way regardless of SQLAlchemy version. See Modifying and Querying Data for more information about queries.