Flask and SQLAlchemy: init_app

I’ve been recently refactoring my Flask application and one of the most important changes I’ve made was an application factory. You can find out many different (but similar) syntax constructions to use SQLAlchemy with it. Here one and most common:

from flask import Flask
from flask.ext.sqlalchemy import SQLAlchemy
db = SQLAlchemy()
def create_app():
    app = Flask(__name__)
    db.init_app(app)
    db.create_all()
    return app

However, you might stumple upon this exception: RuntimeError: application not registered on db instance and no application bound to current context, perhaps in your tests on during db.create_all().

Documentation isn’t that clear on that matter. After a lot of searching I found out three options to solve this issue.

Option #1

Create your db object with app as a parameter:

def create_app():
    app = Flask(__name__)
    db = SQLAlchemy(app)
    db.create_all()
    return app

The drawback is that you don’t have a global db object and thus your blueprints won’t work. And you won’t be able to easily create models.

(BTW: setting global db and then db = SQLAlchemy(app) did not work for me.)

Option #2

Assign created application to db.app:

db = SQLAlchemy()
def create_app():
    app = Flask(__name__)
    db.init_app()
    db.app = app
    db.create_all()
    return app

The possible drawback: you might mess something up in db’s internals. You don’t want that, do you?

Option #3

Call db.create_all() (that’s the thing we had troubles with) within application context:

db = SQLAlchemy()
def create_app():
    app = Flask(__name__)
    db.init_app(app)
    with app.test_request_context():
        db.create_all()
    return app

The drawback: it’s test_request_context (test!). But, after all, it works fine for me. And all my tests pass! ;)

Use any method you want.

If you had this issue and you solved it another way, please let me know in the comments.

Additional tips

You will need to import models themselves before issuing db.create_all:

with app.test_request_context():
    from application.models import Post
    db.create_all()

It’s a good idea to keep your SQLAlchemy object instance in separate file, to avoid circular imports:

# application/database.py
from flask.ext.sqlalchemy import SQLAlchemy
db = SQLAlchemy()

# application factory
from application.database import db
def create_app():
    ...
    db.init_app(app)
    ...

# application/models.py
from application.database import db
class Post(db.Model):
    ...