How to have default/pre-existing rows as ready to use objects using SQLAlchemy?

Question

I'm developing a system using SQLALchemy to persist my data. I'd like to have objects of my default rows so I could use then when creating new instances.

This would be very good for me since some tables are supposed to have some default values. Imagine a simple relation between trow tables (order and order_status). When inserting values in order, it would be nice to have an easy way to use some of this default values already in order_status without having to query for them every time.

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


engine = create_engine('sqlite:///:memory:')

Base = declarative_base()

class Order(Base):
    __tablename__ = 'order'

    id = Column(Integer, primary_key=True)
    order_status_id = Column(Integer, ForeignKey('order_status.id'))
    value = Column(Integer)

    order_status = relationship('OrderStatus', lazy=True, uselist=False)

class OrderStatus(Base):
    __tablename__ = 'order_status'

    id = Column(Integer, primary_key=True)
    name = Column(String(32))

Base.metadata.create_all(engine)

Session = sessionmaker(bind=engine)
session = Session()

finished = OrderStatus(name='Finished')
session.add(finished)
session.commit()

For example, I'd like to create a new instance of Order using something like:

order = Order(value=10, order_status=OrderStatus.FINISHED)

Instead of:

finished_status = session.query(OrderStatus).filter_by(name='Finished').first()
order = Order(value=10, order_status=finished_status)

It does not need to be implemented exactly like my example, but I'd like something similar

Answer 1

You should take a look at the association_proxy : https://docs.sqlalchemy.org/en/13/orm/extensions/associationproxy.html

The example pretty much fits your question. Your Order could finally look somewhat like this (obviously not copy&paste'able):

class OrderStatus(Base):
    ...

class Order(Base):
    __tablename__ = 'order'

    id = Column(Integer, primary_key=True)
    order_status_id = Column(Integer, ForeignKey('order_status.id'))
    value = Column(Integer)

    _order_status = relationship('OrderStatus', lazy=True, uselist=False)
    order_status = association_proxy('_order_status', 'name', creator=get_or_create_order_status)

def get_or_create_order_status(name):
    # get or create a order status by name

In get_or_create_order_status you probably have to go back to querying (at least initially to eg fill a cache).

If the OrderStatus is actually as simple as in your question, maybe an enum would do the trick as well. That would obviously be much easier to manage.