Introduction into Contexts¶
If you are planning on using only one application you can largely skip
this chapter. Just pass your application to the
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.
from flask import Flask from flask_sqlalchemy import SQLAlchemy db = SQLAlchemy() def create_app(): app = Flask(__name__) db.init_app(app) 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(...) db.session.add(user) db.session.commit()
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())