Chapter 7. Querying and Updating at the ORM Level
This chapter introduces the SQLAlchemy Session
object. You will learn how to use the Session to
perform queries and updates of mapped classes, as well as how to customize
the Session class and create a “contextual” session
that simplifies session management.
The SQLAlchemy ORM Session Object
SQLAlchemy manages all querying and updating of objects in the ORM with
Session objects. The Session
is responsible for implementing the unit of work pattern of
synchronization between in-memory objects and database tables. Sessions
also provide a rich interface for querying the database based on object
attributes rather than the underlying SQL database structure.
Creating a Session
The first step in creating a session is to obtain a
Session object from SQLAlchemy. One way to do
this is to directly instantiate the
sqlalchemy.orm.session.Session class. However,
this constructor for the SQLAlchemy-provided
Session has a number of keyword arguments, making
instantiating Sessions in this manner verbose and
tedious. In order to alleviate this burden, SQLAlchemy provides the
sessionmaker() function, which returns a subclass of
orm.session.Session with default arguments set
for its constructor.
Once you have this customized Session
class, you can instantiate it as many times as necessary in your
application without needing to retype the keyword arguments (which in
many applications will not change between Session instantiations). If you wish to override ...
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