8.16. SQLAlchemy Connection Session

  • Session manages persistence operations for ORM-mapped objects

  • 1.0 style (legacy)

  • 2.0 style (with context managers)

session

Manages persistence operations for ORM-mapped objects

8.16.1. SetUp

>>> from sqlalchemy import create_engine
>>>
>>>
>>> DATABASE = 'sqlite:///:memory:'
>>> engine = create_engine(DATABASE)

8.16.2. Sessionmaker

  • Session manages persistence operations for ORM-mapped objects

sessionmaker acts as a factory for Session objects in the same way as an Engine acts as a factory for Connection objects. In this way it also includes a sessionmaker.begin() method, that provides a context manager which both begins and commits a transaction, as well as closes out the Session when complete, rolling back the transaction if any errors occur.

Factory function sessionmaker() will return a class. In order to create a session this class has to be called. There are several ways how to do that. You can either capture the class from session maker, instantiate it and then assign to identifier (variable) or you can do it step by step having intermediate objects.

In order to use sessionmaker() you have to import it:

>>> from sqlalchemy.orm import sessionmaker

8.16.3. 1.x Style

To crate a session object simply use the sessionmaker() factory passing (binding) an engine instance:

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

Or you can simplify the expression by calling class right away:

>>> session = sessionmaker(bind=engine)()

Or using a bit more verbose, but explicit syntax:

>>> session = sessionmaker(bind=engine).__call__()

8.16.4. 2.x Style

Context manager on with block exit will commit transaction and close the session automatically:

>>> from sqlalchemy.orm import Session
>>>
>>> with Session(engine) as session:
...     session.add(object1)
...     session.add(object2)
...     session.commit()

8.16.5. Transaction

>>> with Session(engine) as session:
...     session.begin()
...     try:
...         session.add(some_object)
...         session.add(some_other_object)
...     except:
...         session.rollback()
...         raise
...     else:
...         session.commit()

>>> with Session(engine) as session:
...     with session.begin():
...       session.add(some_object)
...       session.add(some_other_object)

>>> with Session(engine) as session, session.begin():
...     session.add(some_object)
...     session.add(some_other_object)

8.16.6. Example

>>> from sqlalchemy import create_engine
>>> from sqlalchemy.orm import sessionmaker
>>>
>>>
>>> DATABASE = 'sqlite:///:memory:'
>>>
>>> engine = create_engine(DATABASE)
>>> session = sessionmaker(bind=engine)
>>>
>>> with session.begin() as db:
...     result = db.execute('SELECT * FROM astronauts').all()
...
[(1, 'Melissa', 'Lewis', 805766400000),
 (2, 'Rick', 'Martinez', 822182400000),
 (3, 'Alex', 'Vogel', 784857600000),
 (4, 'Chris', 'Beck', 933552000000),
 (5, 'Beth', 'Johanssen', 822182400000),
 (6, 'Mark', 'Watney', 781920000000)]