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Lazy SQLAlchemy setup

Posted by Dan Jacob on 2010-05-27 @ 20:15 and filed in Database

If you are using the new Flask configuration together with the application factory pattern, one thing you will want to do if using SQLAlchemy is initialize a SQLAlchemy session for different requirements. For example, for unit tests you don't want to use the production database.

Furthermore you may need to use a SQLAlchemy session outside the request scope, for example in the shell.

The scoped_session function, which provides a thread-safe session, expects a factory function. Normally we would use sessionmaker:

from sqlalchemy import create_engine
from sqlalchemy.orm import scoped_session, sessionmaker

engine = create_engine("sqlite:///myapp.db")

db_session = scoped_session(sessionmaker(bind=engine))

In order to initialize SQLAlchemy dynamically however we need to pass in a factory function that does not require a ready engine instance. For this we can use create_session:

from sqlalchemy import create_engine
from sqlalchemy.orm import scoped_session, create_session

engine = None

db_session = scoped_session(lambda: create_session(bind=engine))

We then need a function to create the engine when needed:

def init_engine(uri, **kwargs):
    global engine
    engine = create_engine(uri, **kwargs)
    return engine

Provided you call init_engine first you can then use db_session thereafter, as the scoped_session will bind the session to the current value of engine.

You can call init_engine in your application factory function:

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

Now you can use the db_session anywhere in your application, as long as you first call create_app.

This snippet by Dan Jacob can be used freely for anything you like. Consider it public domain.