Seamless Polymorphic Inheritance for Django Models
Polymorphic Models for Django
Quick Start, Docs, Contributing
- What is django_polymorphic good for?
- Quickstart, or the complete Installation and Usage Docs
- Release Notes, News and Discussion (Google Group) or Changelog
- Download from GitHub or Bitbucket, or as TGZ or ZIP
- Improve django_polymorphic, report issues, discuss, patch or fork (GitHub, Bitbucket, Group, Mail)
What is django_polymorphic good for?
Let’s assume the models ArtProject and ResearchProject are derived from the model Project, and let’s store one of each into the database:
>>> Project.objects.create(topic="Department Party") >>> ArtProject.objects.create(topic="Painting with Tim", artist="T. Turner") >>> ResearchProject.objects.create(topic="Swallow Aerodynamics", supervisor="Dr. Winter")
If we want to retrieve all our projects, we do:
Using django_polymorphic, we simply get what we stored:
[ <Project: id 1, topic "Department Party">, <ArtProject: id 2, topic "Painting with Tim", artist "T. Turner">, <ResearchProject: id 3, topic "Swallow Aerodynamics", supervisor "Dr. Winter"> ]
Using vanilla Django, we get incomplete objects, which is probably not what we wanted:
[ <Project: id 1, topic "Department Party">, <Project: id 2, topic "Painting with Tim">, <Project: id 3, topic "Swallow Aerodynamics"> ]
It’s very similar for ForeignKeys, ManyToManyFields or OneToOneFields.
In general, the effect of django_polymorphic is twofold:
On one hand it makes sure that model inheritance just works as you expect, by simply ensuring that you always get back exactly the same objects from the database you stored there - regardless how you access them, making model inheritance much more “pythonic”. This can save you a lot of unpleasant workarounds that tend to make your code messy, error-prone, and slow.
On the other hand, together with some small API additions to the Django ORM, django_polymorphic enables a much more expressive and intuitive programming style and also very advanced object oriented designs that are not possible with vanilla Django.
Fortunately, most of the heavy duty machinery that is needed for this functionality is already present in the original Django database layer. Django_polymorphic adds a rather thin layer above that in order to make real OO fully automatic and very easy to use.
There is a catch however, which applies to concrete model inheritance in general: Current DBM systems like PostgreSQL or MySQL are not very good at processing the required sql queries and can be rather slow in many cases. Concrete benchmarks are forthcoming (please see discussion forum).
This is a V1.0 Beta/Testing Release
The release contains a considerable amount of changes in some of the more critical parts of the software. It’s intended for testing and development environments and not for production environments. For these, it’s best to wait a few weeks for the proper V1.0 release, to allow some time for any potential problems to turn up (if they exist).
If you encounter any problems or have suggestions regarding the API or the changes in this beta, please post them in the discussion group or open an issue on GitHub or BitBucket (or send me an email).
Django_polymorphic uses the same license as Django (BSD-like).
API Changes & Additions
November 11 2010, V1.0 API Changes
extra() queryset method
.extra() has been re-implemented. Now it’s polymorphic by default and works (nearly) without restrictions (please see docs). This is a (very) incompatible API change regarding previous versions of django_polymorphic. Support for the polymorphic keyword parameter has been removed. You can get back the non-polymorphic behaviour by using ModelA.objects.non_polymorphic().extra().
No Pretty-Printing of Querysets by default
In order to improve compatibility with vanilla Django, printing quersets (__repr__ and __unicode__) does not use django_polymorphic’s pretty printing by default anymore. To get the old behaviour when printing querysets, you need to replace your model definition:
>>> class Project(PolymorphicModel):
>>> class Project(PolymorphicModel, ShowFieldType):
The mixin classes for pretty output have been renamed:
ShowFieldTypes, ShowFields, ShowFieldsAndTypes
ShowFieldType, ShowFieldContent and ShowFieldTypeAndContent
(the old ones still exist for compatibility)
Pretty-Printing Output Format Changed
ShowFieldContent and ShowFieldTypeAndContent now use a slightly different output format. If this causes too much trouble for your test cases, you can get the old behaviour back (mostly) by adding polymorphic_showfield_old_format = True to your model definitions. ShowField... now also produces more informative output for custom primary keys.
The polymorphic_dumpdata management command is not needed anymore and has been disabled, as the regular Django dumpdata command now automatically works correctly with polymorphic models (for all supported versions of Django).
Running the Test suite with Django 1.3
Django 1.3 requires python manage.py test polymorphic instead of just python manage.py test.
November 01 2010, V1.0 API Additions
.non_polymorphic() queryset member function added. This is preferable to using .base_objects..., as it just makes the resulting queryset non-polymorphic and does not change anything else in the behaviour of the manager used (while .base_objects is just a different manager).
.get_real_instances() has been elevated to an official part of the API. It allows you to turn a queryset or list of base objects into a list of the real instances. This is useful if e.g. you use ModelA.objects.non_polymorphic().extra(...) and then want to transform the result to its polymorphic equivalent:
>>> qs = ModelA.objects.all().non_polymorphic() >>> real_objects = qs.get_real_instances()
is equivalent to:
>>> real_objects = ModelA.objects.all()
Instead of qs.get_real_instances(), ModelA.objects.get_real_instances(qs) may be used as well. In the latter case, qs may be any list of objects of type ModelA.
translate_polymorphic_Q_object (see DOCS)
February 22 2010, Installation Note
The django_polymorphic source code has been restructured and as a result needs to be installed like a normal Django App - either via copying the “polymorphic” directory into your Django project or by running setup.py. Adding ‘polymorphic’ to INSTALLED_APPS in settings.py is still optional, however.
The file polymorphic.py cannot be used as a standalone extension module anymore (as is has been split into a number of smaller files).
Importing works slightly different now: All relevant symbols are imported directly from ‘polymorphic’ instead from ‘polymorphic.models’:
# new way from polymorphic import PolymorphicModel, ... # old way, doesn't work anymore from polymorphic.models import PolymorphicModel, ...
January 26 2010: Database Schema Change
The update from January 26 changed the database schema (more info in the commit-log). Sorry for any inconvenience. But this should be the final DB schema now.