database_usage.md

Database Usage

Introduction

This document provides information about programmatically accessing the database.

For information about setting up the database, see Database Setup

Context Managers

As recommended in the SQL Alchemy documentation (see the "When do I construct a Session, when do I commit it, and when do I close it?" section) database access is performed within a Python "context manager".

There are two context manager functions in the "dwetl/init.py_" file for accessing the database:

• dwetl.database_session
• dwetl.test_database_session

The "dwetl.database_session" is used by the application when the changes should actually be stored in the database. All changes made within the session are committed, unless an exception is raised, in which case all the changes are rolled back.

The "dwetl.test_database_session" is used by unit/integration tests. Any changes made within the session are automatically rolled back when the session exits.

Both the "dwetl .database_session" and "dwetl.test_database_session" are used in the same way:

import dwetl

...

with dwetl.database_session() as session:
  # Do database access

When writing a record to the database, an explicit "session.commit()" should not be used.