Writing re-usable, decoupled models (multi-app architecture)

[leonidumanskiy]

I am writing a complex web application that consists of a multiple flask apps, as well as non-flask Python apps (internal scheduler, etc).
I need to write models that I can re-use across multiple Flask apps, as well as non-flask python code using pure SA. However, I don't want to lose any benefits that Flask-SQLAlchemy gives you (when possible).

My first approach was to use declarative_base and it somewhat works:

from flask_sqlalchemy import declarative_base, DefaultMeta

Base = declarative_base(bind=user_db_conn_string, metaclass=DefaultMeta)

class User(Base):
    __bind_key__ = "user_db"
    name = Column(String(128), nullable=False)
    ....

However, multi-database binds don't seem to work properly. When I run

db.session.query(User).all()

It ignores the bind/bind key and runs it against the default database (and fails).

What should be correct approach here?

[ThiefMaster]

Personally I just use Flask-SQLAlchemy everywhere. Even if it's not a webapp, you can simply wrap your code with a flask app context - all you need is to call db.init_app(app) and previously instantiate the app and set the sqlalchemy-related config.

[davidism]

See #250 for previous work on this and https://stackoverflow.com/a/33127999/400617 for an example of what ThiefMaster is talking about.

[leonidumanskiy]

Hi @davidism

I am really sorry but unfortunately, neither of those two links answer my question.

Example given in the end of #250 discussion does not work with a multi-database setup. Specifically, models do not bind to the correct engine using with __bind_key__ and I could not figure out how to make it bind properly.

Second link and solution provided by ThiefMaster allows to use models across multiple flask applications but it does not solve for non-flask environment. Shipping flask and flask-sqlalchemy with non-flask applications is barely a workaround and does not scale.

Imagine the following scenario - you have a non-flask codebase, developed by another team using SQLAlchemy, and you are developing a simple Flask-based frontend for it. Asking them to couple their architecture to flask-sqlalchemy and flask seems like a really bad idea.

[davidism]

Please use Stack Overflow for questions about your own code. This tracker is for issues related to the project itself. Be sure to include a minimal, complete, and verifiable example.