Introduction into Contexts

If you are planning on using only one application you can largely skip this chapter. Just pass your application to the SQLAlchemy constructor and you’re usually set. However if you want to use more than one application or create the application dynamically in a function you want to read on.

If you define your application in a function, but the SQLAlchemy object globally, how does the latter learn about the former? The answer is the init_app() function:

from flask import Flask
from flask_sqlalchemy import SQLAlchemy

db = SQLAlchemy()

def create_app():
    app = Flask(__name__)
    return app

What it does is prepare the application to work with SQLAlchemy. However that does not now bind the SQLAlchemy object to your application. Why doesn’t it do that? Because there might be more than one application created.

So how does SQLAlchemy come to know about your application? You will have to setup an application context. If you are working inside a Flask view function or a CLI command, that automatically happens. However, if you are working inside the interactive shell, you will have to do that yourself (see Creating an Application Context).

If you try to perform database operations outside an application context, you will see the following error:

No application found. Either work inside a view function or push an application context.

In a nutshell, do something like this:

>>> from yourapp import create_app
>>> app = create_app()
>>> app.app_context().push()

Alternatively, use the with-statement to take care of setup and teardown:

def my_function():
    with app.app_context():
        user = db.User(...)

Some functions inside Flask-SQLAlchemy also accept optionally the application to operate on:

>>> from yourapp import db, create_app
>>> db.create_all(app=create_app())