When scattering Flask Models, RuntimeError: ‘application not registered on db’ was raised

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This has to do with Flask’s application context. When initialized with db.init_app(app), Flask-SQLAlchemy doesn’t know which app is the “current” app (remember, Flask allows for multiple apps in the same interpreter). You could have multiple apps using the same SQLAlchemy instance in the same process, and Flask-SQLAlchemy would need to know which is the “current” one (due to Flask’s context local nature of everything).

If you need to do this during runtime, you must explicitly say which app is the “current” app for all calls. You can do this by changing your code to use a with app.app_context() block:

def create_app():
    app = flask.Flask("app")
    app.config['SQLALCHEMY_DATABASE_URI'] = 'sqlite://'
    app.register_blueprint(api)
    db.init_app(app)
    with app.app_context():
        # Extensions like Flask-SQLAlchemy now know what the "current" app
        # is while within this block. Therefore, you can now run........
        db.create_all()

    return app

If you are writing a standalone script that needs the app context, you can push the context at the beginning rather than putting everything in a with block.

create_app().app_context().push()

If you write a command for Flask’s cli the command will automatically have access to the context.

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