Using sqlalchemy-continuum with flask-sqlalchemy and flask-migrate

Question

I'm trying to get sqlalchemy-continuum to work alongside flask-sqlalchemy and flask-migrate. My __init__.py file looks like this:

import os

from flask import Flask

def create_app():
    """Create and configure an instance of the Flask application."""
    app = Flask(__name__, instance_relative_config=True)

    app.config.from_mapping(
        SQLALCHEMY_DATABASE_URI='postgres+psycopg2://{}:{}@{}:{}/{}'.format(
            os.environ['POSTGRES_USER'],
            os.environ['POSTGRES_PASSWORD'],
            os.environ['POSTGRES_HOST'],
            os.environ['POSTGRES_PORT'],
            os.environ['POSTGRES_DB']
        ),
        SQLALCHEMY_TRACK_MODIFICATIONS=False
    )

    try:
        os.makedirs(app.instance_path)
    except OSError:
        pass

    from .models import db, migrate
    db.init_app(app)
    migrate.init_app(app, db)

    return app

My models.py file looks like this:

import sqlalchemy
from flask_sqlalchemy import SQLAlchemy
from flask_migrate import Migrate
from sqlalchemy_continuum import make_versioned


db = SQLAlchemy()
migrate = Migrate()

make_versioned(user_cls=None)

class User(db.Model):
    __versioned__ = {}
    __tablename__ = 'user'

    id = db.Column(db.Integer, primary_key=True)
    username = db.Column(db.String(80), unique=True, nullable=False)
    email = db.Column(db.String(120), unique=True, nullable=False)
    password = db.Column(db.String(20), unique=True, nullable=False)

    def __repr__(self):
        return '<User {} - {}>'.format(self.username, self.email)

sqlalchemy.orm.configure_mappers()

I then run the following flask-migrate commands to initialise and migrate the database:

flask db init
flask db migrate
flask db upgrade

The output of the flask db upgrade command seems to show the correct tables being created:

INFO  [alembic.runtime.migration] Context impl PostgresqlImpl.
INFO  [alembic.runtime.migration] Will assume transactional DDL.
INFO  [alembic.autogenerate.compare] Detected added table 'transaction'
INFO  [alembic.autogenerate.compare] Detected added table 'user'
INFO  [alembic.autogenerate.compare] Detected added table 'user_version'
INFO  [alembic.autogenerate.compare] Detected added index 'ix_user_version_end_transaction_id' on '['end_transaction_id']'
INFO  [alembic.autogenerate.compare] Detected added index 'ix_user_version_operation_type' on '['operation_type']'
INFO  [alembic.autogenerate.compare] Detected added index 'ix_user_version_transaction_id' on '['transaction_id']'

In the python shell I can do the following:

>>> from test_flask.__init__ import create_app
>>> from test_flask.models import db, User

>>> app = create_app()
>>> with app.app_context():
...     user = User(username='devuser', email='[email protected]', 
password='devpassword')
...     db.session.add(user)
...     db.session.commit()

This seems to work fine but when I attempt to access an element in the versions attribute using:

>>> user.versions[0]

I get the following error:

Traceback (most recent call last):
File "<stdin>", line 1, in <module>
File "/usr/local/lib/python3.6/site-packages/sqlalchemy/orm/dynamic.py", line 254, in __getitem__
  attributes.PASSIVE_NO_INITIALIZE).indexed(index)
File "/usr/local/lib/python3.6/site-packages/sqlalchemy/orm/dynamic.py", line 359, in indexed
  return list(self.added_items)[index]
IndexError: list index out of range

The command:

>>> user.versions

returns:

<sqlalchemy.orm.dynamic.AppenderQuery object at 0x7f6515d3a898>

This doesn't seem to be the expected behaviour of the versions attribute, as specified in the sqlalchemy-continuum docs. Any ideas as to what I've done wrong?

Answer 1

This error could happen in two cases, at least :

• you didn't commit after some UPDATE or INSERT
• or you're not in the app context

In your example from the flask shell, all statements should be inside the context block :

>>> with app.app_context():
...     user = User(username='devuser2', email='[email protected]', password='devpassword2')
...     db.session.add(user)
...     db.session.commit()
...     user.versions[0].username
...     user.username='devuser_fixed'
...     db.session.commit()    
...     user.versions[1].username

#'devuser'
#'devuser_fixed'

Outside the context, user is still living, the variable stays in memory, but the session connected to the database is lost.

---

Note : if the 2nd commit is missing, user.versions[1] will be available only in the actual context session. So if you exit this context and check user.versions[1], you will face the same error.