How to fix my approach to use the same models in vanilla SQLAlchemy and Flask-SQLAlchemy?

Question

I came across several approaches on how to use the vanilla SQLAlchemy models in Flask-SQLAlchemy.

It works like a charm to use models that inherit from Base

Answer 1

I found a good solution inspired by a Factory pattern and Declarative Mixins as mentioned in the SQLAlchemy docs.

For complex multi-level inheritance scenarios a different approach is needed, using @declared_attr.cascading.

from sqlalchemy import create_engine
from sqlalchemy.orm import sessionmaker
from sqlalchemy import Column, Integer, String
from sqlalchemy import MetaData

from sqlalchemy.ext.declarative import declarative_base
from flask_sqlalchemy import SQLAlchemy

SQLALCHEMY_DATABASE_URI = 'sqlite:///' + '/tmp/test_app.db'
engine = create_engine(SQLALCHEMY_DATABASE_URI, echo=True)

# for vanilla
Base = declarative_base()

# for Flask (import from app once initialized)
db = SQLAlchemy()


class MachineMixin:

    __tablename__ = 'machine'
    id = Column(Integer, primary_key=True)
    name = Column(String(100))
    status = Column(Integer)


class ModelFactory:

    @staticmethod
    def create(which_model, which_parent):

        if which_parent == 'flask_sqlalchemy':

            parent = db.Model

        elif which_parent == 'pure_sqlalchemy':

            parent = Base

        # now use type() to interit, fill __dict__ and assign a name
        obj = type(which_model.__name__ + '_' + which_parent,
                    (which_model, parent),
                    {})
        return obj


test_scenario = 'pure_sqlalchemy' # 'flask_sqlalchemy'

Machine = ModelFactory.create(MachineMixin, test_scenario)

if test_scenario == 'flask_sqlalchemy':

    db.metadata.drop_all(bind=engine)
    db.metadata.create_all(bind=engine)

elif test_scenario == 'pure_sqlalchemy':

    Base.metadata.drop_all(bind=engine)
    Base.metadata.create_all(bind=engine)


Session = sessionmaker(bind=engine)
session = Session()
session.add(Machine(name='Bob', status=1))
session.commit()