Essential SQLAlchemy, 2nd Edition

Chapter 8. Understanding the Session and Exceptions

In the previous chapter, we did a lot of work with the session, and we avoided doing anything that could result in an exception. In this chapter, we will learn a bit more about how our objects and the SQLAlchemy session interact. We’ll conclude this chapter by purposely performing some actions incorrectly so that we can see the types of exceptions that occur and how we should respond to them. Let’s start by learning more about the session. First, we’ll set up an in-memory SQLite database using the tables from Chapter 6:

from sqlalchemy import create_engine
from sqlalchemy.orm import sessionmaker

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

Session = sessionmaker(bind=engine)

session = Session()

from datetime import datetime

from sqlalchemy import (Table, Column, Integer, Numeric, String, DateTime,
                        ForeignKey, Boolean)
from sqlalchemy.ext.declarative import declarative_base
from sqlalchemy.orm import relationship, backref


Base = declarative_base()


class Cookie(Base):
    __tablename__ = 'cookies'

    cookie_id = Column(Integer, primary_key=True)
    cookie_name = Column(String(50), index=True)
    cookie_recipe_url = Column(String(255))
    cookie_sku = Column(String(55))
    quantity = Column(Integer())
    unit_cost = Column(Numeric(12, 2))

    def __init__(self, name, recipe_url=None, sku=None, quantity=0,
                 unit_cost=0.00):
        self.cookie_name = name
        self.cookie_recipe_url = recipe_url
        self.cookie_sku = sku
        self.quantity = quantity
        self.unit_cost = unit_cost ...