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sabridge API

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django-sabridge - SQLAlchemy access to Django models

Motivation

Django’s ORM is wonderful and easy to use. When the standard ORM operations are insufficient for an application, Django provides multiple methods for more directly interacting with the database, including django.db.models.Manager.raw() and django.db.connection.cursor(). However, using these methods can easily lead to vendor lock-in. Additionally, programmatically building SQL is a difficult task (especially to do so securely).

SQLAlchemy offers an excellent SQL rendering engine, which allows programmatic generation of SQL. By exposing Django models via SQLAlchemy’s Expression Language, a developer can build extremely complex queries while retaining database-independence (vendor-specific features are still available).

Other efforts have aimed to replace Django’s ORM with SQLAlchemy’s ORM. django-sabridge instead leaves Django’s ORM in place, while allowing SQLAlchemy Expression Language to easily access those Django models.

django-sabridge addresses a specific need. It may not be the ideal solution, so please contribute better approaches. Please also be aware of the Caveats.

Usage

To demonstrate sabridge, we will access django.contrib.auth.models.User through SQLAlchemy.

First, import and initialize the sabridge.Bridge:

>>> from sabridge import Bridge
>>> bridge = Bridge()

We use the model’s class to obtain the SQLAlchemy version of the table:

>>> from django.contrib.auth.models import User
>>> table = bridge[model]

The sabridge.Bridge returns an instance of sqlalchemy.schema.Table. If we write data in Django, we can then view that data via SQLAlchemy:

>>> User.objects.create(username='alice')
>>> result = list(table.select().execute())
>>> len(result)
1
>>> result[0][table.c.username]
u'alice'

Caveats

Transactions

sabridge does not re-use Django’s connection to the database, thus if executing in a transaction, any data modified by either Django or SQLAlchemy will not be visible to the other, until the transaction is committed.

Practically, this means that any test cases that uses both Django and SQLAlchemy will have to inherit from django.test.TransactionTestCase instead of the more typical django.test.TestCase. The TransactionTestCase does not wrap each test in a transaction, thus the data modified by SQLAlchemy and Django is not isolated. Unfortunately, the TransactionTestCase is significantly slower than the normal TestCase. Refer to the TransactionTestCase documentation.

Performance

sabridge uses SQLAlchemy’s reflection (autoload=True) to discover the schema of the requested Django model. Efforts are made to reduce the number of times introspection occurs, but a user of django-sabridge should make sure that it fits within any performance requirements.