Using Flask SQLAlchemy models with a regular SQLAlchemy

Question

I have an existing model that I can't change, written in Flask-SQLAlchemy. I'm writing another app that uses the same model, but without the need for Flask, therefore I'm working with the regular SQLAlchemy module. Unfortunately, I'm getting a lot of: 'AttributeError: module 'DB' has no attribute 'Model' for all kind of attributes - such as Column, Integer, etc Is there a way to use Flask-SQLAlchemy with a regular SQLAlchemy app?

There is an example of one of my Model Class:

class Table_name(Model):
    __tablename__ = 'table_name'
    id = db.Column(db.INTEGER, primary_key=True)
    field1 = db.Column(db.INTEGER, db.ForeignKey('table1.id'), nullable=False)
    field2 = db.Column(db.TEXT, nullable=False)
    field3 = db.Column(db.INTEGER, db.ForeignKey('table2.id'), nullable=False)
    time = db.Column(db.TIMESTAMP, nullable=False)

Unfortunately I can't change them

Flask-SQLAlchemy thin wrapper binding for sqlalchemy in a flask. You could use sqlalchemy without flask-sqlalchemy. Without looking code or your effort it is hard to say. — Debendra
– Debendra, Commented Jan 30, 2020 at 9:17
I added example of one of my model class, Unfortunately the model is legacy, therefore, I can't change it — MrAirman
– MrAirman, Commented Jan 30, 2020 at 9:50

Ken Kinder · Accepted Answer · 2020-01-30 09:56:03Z

I've had the same dilemma. If it's a small or one-off, you can hack in a Flask app object without really using it, like so:

from flask_sqlalchemy import SQLAlchemy

throw_away_app = Flask(__name__)
db = SQLAlchemy(throw_away_app)
with self.throw_away_app.app_context():
    (perform your db operation)

That works relatively well for simple things like scripts. But if you're sharing a model across multiple projects and you simply cannot alter the existing Flask project, it's unfortunately probably not a good solution.

If that's the case, and you simply cannot alter the existing codebase at all, it probably makes sense to create a new model class and connect to the existing database using vanilla SQLAlchemy.

BTW, for any future programmers wondering how to get some of the benefits of Flask-SQLAlchemy without Flask, consider: sqlalchemy-mixins.

Debendra · Accepted Answer · 2020-03-02 07:45:01Z

from sqlalchemy import create_engine
from sqlalchemy import Column, Integer, String, Text, TIMESTAMP, ForeignKey
from sqlalchemy.ext.declarative import declarative_base
from sqlalchemy.orm import sessionmaker


Base = declarative_base()

#  connect in memory sqlite database or you can connect your own database
engine = create_engine('sqlite:///:memory:', echo=True)

# create session and bind engine
Session = sessionmaker(bind=engine)


class Table_name(Base):
    __tablename__ = 'table_name'
    id = Column(Integer, primary_key=True)
    field1 = Column(Integer, ForeignKey('table1.id'), nullable=False)
    field2 = Column(Text, nullable=False)
    field3 = Column(Integer, ForeignKey('table2.id'), nullable=False)
    time = Column(TIMESTAMP, nullable=False)

table = Table_name(field1=1, fi....)
session.add(table)

Now you can use your ORM as usual like flask-sqlalchemy .

Docs: https://docs.sqlalchemy.org/en/13/orm/tutorial.html

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