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Polymorphic Models for Django
=============================
.. contents:: Table of Contents
:depth: 1
Quickstart
===========
Install
-------
After uncompressing (if necessary), in the directory "...django_polymorphic",
execute (on Unix-like systems)::
sudo python setup.py install
Make Your Models Polymorphic
----------------------------
Use ``PolymorphicModel`` instead of Django's ``models.Model``, like so::
from polymorphic import PolymorphicModel
class Project(PolymorphicModel):
topic = models.CharField(max_length=30)
class ArtProject(Project):
artist = models.CharField(max_length=30)
class ResearchProject(Project):
supervisor = models.CharField(max_length=30)
All models inheriting from your polymorphic models will be polymorphic as well.
Create some objects
-------------------
>>> Project.objects.create(topic="Office Meeting")
>>> ArtProject.objects.create(topic="Painting with Tim", artist="T. Turner")
>>> ResearchProject.objects.create(topic="Swallow Aerodynamics", supervisor="Dr. Winter")
Get polymorphic query results
-----------------------------
>>> Project.objects.all()
[ <Project: id 1, topic "John's Gathering">,
<ArtProject: id 2, topic "Painting with Tim", artist "T. Turner">,
<ResearchProject: id 3, topic "Swallow Aerodynamics", supervisor "Dr. Winter"> ]
use ``instance_of`` or ``not_instance_of`` for narrowing the result to specific subtypes:
>>> Project.objects.instance_of(ArtProject)
[ <ArtProject: id 2, topic "Painting with Tim", artist "T. Turner"> ]
>>> Project.objects.instance_of(ArtProject) | Project.objects.instance_of(ResearchProject)
[ <ArtProject: id 2, topic "Painting with Tim", artist "T. Turner">,
<ResearchProject: id 3, topic "Swallow Aerodynamics", supervisor "Dr. Winter"> ]
Polymorphic filtering: Get all projects where Mr. Turner is involved as an artist
or supervisor (note the three underscores):
>>> Project.objects.filter( Q(ArtProject___artist = 'T. Turner') | Q(ResearchProject___supervisor = 'T. Turner') )
[ <ArtProject: id 2, topic "Painting with Tim", artist "T. Turner">,
<ResearchProject: id 4, topic "Color Use in Late Cubism", supervisor "T. Turner"> ]
This is basically all you need to know, as django_polymorphic mostly
works fully automatic and just delivers the expected ("pythonic") results.
Note: In all example output, above and below, for a nicer and more informative
output the ``ShowFieldType`` mixin has been used (documented below).
List of Features
================
* Fully automatic - generally makes sure that the same objects are
returned from the database that were stored there, regardless how
they are retrieved
* Only on models that request polymorphic behaviour however (and the
models inheriting from them)
* Full support for ForeignKeys, ManyToManyFields and OneToToneFields
* Filtering for classes, equivalent to python's isinstance():
``instance_of(...)`` and ``not_instance_of(...)``
* Polymorphic filtering/ordering etc., allowing the use of fields of
derived models ("ArtProject___artist")
* Support for user-defined custom managers
* Automatic inheritance of custom managers
* Support for user-defined custom queryset classes
* Non-polymorphic queries if needed, with no other change in
features/behaviour
* Combining querysets of different types/models ("qs3 = qs1 | qs2")
* Nice/informative display of polymorphic queryset results
More about Installation / Testing
=================================
Requirements
------------
Django 1.1 (or later) and Python 2.4 or later. This code has been tested
on Django 1.1 / 1.2 / 1.3 and Python 2.4.6 / 2.5.4 / 2.6.4 on Linux.
Included Test Suite
-------------------
The repository (or tar file) contains a complete Django project
that may be used for tests or experiments, without any installation needed.
To run the included test suite, in the directory "...django_polymorphic" execute::
./manage test polymorphic
The management command ``pcmd.py`` in the app ``pexp`` can be used
for quick tests or experiments - modify this file (pexp/management/commands/pcmd.py)
to your liking, then run::
./manage syncdb # db is created in /var/tmp/... (settings.py)
./manage pcmd
Installation
------------
In the directory "...django_polymorphic", execute ``sudo python setup.py install``.
Alternatively you can simply copy the ``polymorphic`` subdirectory
(under "django_polymorphic") into your Django project dir
(e.g. if you want to distribute your project with more 'batteries included').
If you want to run the test cases in `polymorphic/tests.py`, you need to add
``polymorphic`` to your INSTALLED_APPS setting.
Django's ContentType framework (``django.contrib.contenttypes``)
needs to be listed in INSTALLED_APPS (usually it already is).
More Polymorphic Functionality
==============================
In the examples below, these models are being used::
from polymorphic import PolymorphicModel
class ModelA(PolymorphicModel):
field1 = models.CharField(max_length=10)
class ModelB(ModelA):
field2 = models.CharField(max_length=10)
class ModelC(ModelB):
field3 = models.CharField(max_length=10)
Filtering for classes (equivalent to python's isinstance() ):
-------------------------------------------------------------
>>> ModelA.objects.instance_of(ModelB)
.
[ <ModelB: id 2, field1 (CharField), field2 (CharField)>,
<ModelC: id 3, field1 (CharField), field2 (CharField), field3 (CharField)> ]
In general, including or excluding parts of the inheritance tree::
ModelA.objects.instance_of(ModelB [, ModelC ...])
ModelA.objects.not_instance_of(ModelB [, ModelC ...])
You can also use this feature in Q-objects (with the same result as above):
>>> ModelA.objects.filter( Q(instance_of=ModelB) )
Polymorphic filtering (for fields in derived classes)
-----------------------------------------------------
For example, cherrypicking objects from multiple derived classes
anywhere in the inheritance tree, using Q objects (with the
syntax: ``exact model name + three _ + field name``):
>>> ModelA.objects.filter( Q(ModelB___field2 = 'B2') | Q(ModelC___field3 = 'C3') )
.
[ <ModelB: id 2, field1 (CharField), field2 (CharField)>,
<ModelC: id 3, field1 (CharField), field2 (CharField), field3 (CharField)> ]
Combining Querysets / Querysets as "Object Containers"
------------------------------------------------------
Querysets could now be regarded as object containers that allow the
aggregation of different object types, very similar to python
lists - as long as the objects are accessed through the manager of
a common base class:
>>> Base.objects.instance_of(ModelX) | Base.objects.instance_of(ModelY)
.
[ <ModelX: id 1, field_x (CharField)>,
<ModelY: id 2, field_y (CharField)> ]
ManyToManyField, ForeignKey, OneToOneField
------------------------------------------
Relationship fields referring to polymorphic models work as
expected: like polymorphic querysets they now always return the
referred objects with the same type/class these were created and
saved as.
E.g., if in your model you define::
field1 = OneToOneField(ModelA)
then field1 may now also refer to objects of type ``ModelB`` or ``ModelC``.
A ManyToManyField example::
# The model holding the relation may be any kind of model, polymorphic or not
class RelatingModel(models.Model):
many2many = models.ManyToManyField('ModelA') # ManyToMany relation to a polymorphic model
>>> o=RelatingModel.objects.create()
>>> o.many2many.add(ModelA.objects.get(id=1))
>>> o.many2many.add(ModelB.objects.get(id=2))
>>> o.many2many.add(ModelC.objects.get(id=3))
>>> o.many2many.all()
[ <ModelA: id 1, field1 (CharField)>,
<ModelB: id 2, field1 (CharField), field2 (CharField)>,
<ModelC: id 3, field1 (CharField), field2 (CharField), field3 (CharField)> ]
Using Third Party Models (without modifying them)
-------------------------------------------------
Third party models can be used as polymorphic models without
restrictions by subclassing them. E.g. using a third party
model as the root of a polymorphic inheritance tree::
from thirdparty import ThirdPartyModel
class MyThirdPartyBaseModel(PolymorhpicModel, ThirdPartyModel):
pass # or add fields
Or instead integrating the third party model anywhere into an
existing polymorphic inheritance tree::
class MyBaseModel(SomePolymorphicModel):
my_field = models.CharField(max_length=10)
class MyModelWithThirdParty(MyBaseModel, ThirdPartyModel):
pass # or add fields
Non-Polymorphic Queries
-----------------------
If you insert ``.non_polymorphic()`` anywhere into the query chain, then
django_polymorphic will simply leave out the final step of retrieving the
real objects, and the manager/queryset will return objects of the type of
the base class you used for the query, like vanilla Django would
(``ModelA`` in this example).
>>> qs=ModelA.objects.non_polymorphic().all()
>>> qs
[ <ModelA: id 1, field1 (CharField)>,
<ModelA: id 2, field1 (CharField)>,
<ModelA: id 3, field1 (CharField)> ]
There are no other changes in the behaviour of the queryset. For example,
enhancements for ``filter()`` or ``instance_of()`` etc. still work as expected.
If you do the final step yourself, you get the usual polymorphic result:
>>> ModelA.objects.get_real_instances(qs)
[ <ModelA: id 1, field1 (CharField)>,
<ModelB: id 2, field1 (CharField), field2 (CharField)>,
<ModelC: id 3, field1 (CharField), field2 (CharField), field3 (CharField)> ]
About Queryset Methods
----------------------
* ``annotate()`` and ``aggregate()`` work just as usual, with the
addition that the ``ModelX___field`` syntax can be used for the
keyword arguments (but not for the non-keyword arguments).
* ``order_by()`` now similarly supports the ``ModelX___field`` syntax
for specifying ordering through a field in a submodel.
* ``distinct()`` works as expected. It only regards the fields of
the base class, but this should never make a difference.
* ``select_related()`` works just as usual, but it can not (yet) be used
to select relations in derived models
(like ``ModelA.objects.select_related('ModelC___fieldxy')`` )
* ``extra()`` works as expected (it returns polymorphic results) but
currently has one restriction: The resulting objects are required to have
a unique primary key within the result set - otherwise an error is thrown
(this case could be made to work, however it may be mostly unneeded)..
The keyword-argument "polymorphic" is no longer supported.
You can get back the old non-polymorphic behaviour (before V1.0)
by using ``ModelA.objects.non_polymorphic().extra(...)``.
* ``get_real_instances()`` allows you to turn a
queryset or list of base model objects efficiently into the real objects.
For example, you could do ``base_objects_queryset=ModelA.extra(...).non_polymorphic()``
and then call ``real_objects=base_objects_queryset.get_real_instances()``.Or alternatively
.``real_objects=ModelA.objects..get_real_instances(base_objects_queryset_or_object_list)``
* ``values()`` & ``values_list()`` currently do not return polymorphic
results. This may change in the future however. If you want to use these
methods now, it's best if you use ``Model.base_objects.values...`` as
this is guaranteed to not change.
* ``defer()`` and ``only()`` are not yet supported (support will be added
in the future).
Using enhanced Q-objects in any Places
--------------------------------------
Sometimes it would be nice to be able to use the enhanced filter-definitions/Q-objects
outside of polymorphic models/querysets. Example (using ``limit_choices_to``
to filter the selection of objects in the admin)::
class MyModel(models.Model):
somekey = model.ForeignKey(Model2A,
limit_choices_to = Q(instance_of=Model2B) )
``instance_of`` is a django_polymorphic-specific enhancement of Q objects, which the
vanilla django function ``ForeignKey`` cannot process. In such cases you can do::
from polymorphic import translate_polymorphic_Q_object
class MyModel(models.Model):
somekey = model.ForeignKey(Model2A,
limit_choices_to = translate_polymorphic_Q_object( Model2A, Q(instance_of=Model2B) ) )
Nicely Displaying Polymorphic Querysets
---------------------------------------
In order to get the output as seen in all examples here, you need to use the
ShowFieldType class mixin::
from polymorphic import PolymorphicModel, ShowFieldType
class ModelA(ShowFieldType, PolymorphicModel):
field1 = models.CharField(max_length=10)
You may also use ShowFieldContent or ShowFieldTypeAndContent to display
additional information when printing querysets (or converting them to text).
When showing field contents, they will be truncated to 20 characters. You can
modify this behaviour by setting a class variable in your model like this::
class ModelA(ShowFieldType, PolymorphicModel):
polymorphic_showfield_max_field_width = 20
...
Similarly, pre-V1.0 output formatting can be re-estated by using
``polymorphic_showfield_old_format = True``.
Custom Managers, Querysets & Manager Inheritance
================================================
Using a Custom Manager
----------------------
A nice feature of Django is the possibility to define one's own custom object managers.
This is fully supported with django_polymorphic: For creating a custom polymorphic
manager class, just derive your manager from ``PolymorphicManager`` instead of
``models.Manager``. As with vanilla Django, in your model class, you should
explicitly add the default manager first, and then your custom manager::
from polymorphic import PolymorphicModel, PolymorphicManager
class TimeOrderedManager(PolymorphicManager):
def get_query_set(self):
qs = super(TimeOrderedManager,self).get_query_set()
return qs.order_by('-start_date') # order the queryset
def most_recent(self):
qs = self.get_query_set() # get my ordered queryset
return qs[:10] # limit => get ten most recent entries
class Project(PolymorphicModel):
objects = PolymorphicManager() # add the default polymorphic manager first
objects_ordered = TimeOrderedManager() # then add your own manager
start_date = DateTimeField() # project start is this date/time
The first manager defined ('objects' in the example) is used by
Django as automatic manager for several purposes, including accessing
related objects. It must not filter objects and it's safest to use
the plain ``PolymorphicManager`` here.
Manager Inheritance
-------------------
Polymorphic models inherit/propagate all managers from their
base models, as long as these are polymorphic. This means that all
managers defined in polymorphic base models continue to work as
expected in models inheriting from this base model::
from polymorphic import PolymorphicModel, PolymorphicManager
class TimeOrderedManager(PolymorphicManager):
def get_query_set(self):
qs = super(TimeOrderedManager,self).get_query_set()
return qs.order_by('-start_date') # order the queryset
def most_recent(self):
qs = self.get_query_set() # get my ordered queryset
return qs[:10] # limit => get ten most recent entries
class Project(PolymorphicModel):
objects = PolymorphicManager() # add the default polymorphic manager first
objects_ordered = TimeOrderedManager() # then add your own manager
start_date = DateTimeField() # project start is this date/time
class ArtProject(Project): # inherit from Project, inheriting its fields and managers
artist = models.CharField(max_length=30)
ArtProject inherited the managers ``objects`` and ``objects_ordered`` from Project.
``ArtProject.objects_ordered.all()`` will return all art projects ordered
regarding their start time and ``ArtProject.objects_ordered.most_recent()``
will return the ten most recent art projects.
.
Using a Custom Queryset Class
-----------------------------
The ``PolymorphicManager`` class accepts one initialization argument,
which is the queryset class the manager should use. Just as with vanilla Django,
you may define your own custom queryset classes. Just use PolymorphicQuerySet
instead of Django's QuerySet as the base class::
from polymorphic import PolymorphicModel, PolymorphicManager, PolymorphicQuerySet
class MyQuerySet(PolymorphicQuerySet):
def my_queryset_method(...):
...
class MyModel(PolymorphicModel):
my_objects=PolymorphicManager(MyQuerySet)
...
Performance Considerations
==========================
The current implementation is rather simple and does not use any
custom SQL or Django DB layer internals - it is purely based on the
standard Django ORM.
The advantages are that the implementation naturally works on all
supported database management systems, and consists of rather
clean source code which can be easily understood and enhanced.
The disadvantage is that this approach can not deliver the optimum
performance as it introduces additional database queries.
Specifically, the query::
result_objects = list( ModelA.objects.filter(...) )
performs one SQL query to retrieve ``ModelA`` objects and one additional
query for each unique derived class occurring in result_objects.
The best case for retrieving 100 objects is 1 SQL query if all are
class ``ModelA``. If 50 objects are ``ModelA`` and 50 are ``ModelB``, then
two queries are executed. The pathological worst case is 101 db queries if
result_objects contains 100 different object types (with all of them
subclasses of ``ModelA``).
Usually, when Django users create their own polymorphic ad-hoc solution
without a tool like django_polymorphic, this usually results in a variation of ::
result_objects = [ o.get_real_instance() for o in BaseModel.objects.filter(...) ]
which has exceptionally bad performance, as it introduces one additional
SQL query for every object in the result which is not of class ``BaseModel``.
Relative to this, the performance of the current django_polymorphic
implementation is very good.
If your project however needs perfect performance and the current
performance implications of django_polymorphic are not acceptable, then
basically there are the two options of either foregoing of an essential aspect
of object oriented programming or optimizing django_polymorphic.
Foregoing the benefits of this aspect of object oriented programming
for projects that could benefit from it will however usually lead to bloated code,
unnecessary complexity and considerably more of the programmer's time to
create and update the implementation, together with the disadvantages
of a less flexible and less future-proof solution. Throwing a little more
hardware on the problem might be the least expensive solution in most cases.
Possible Optimizations
======================
Django_polymorphic can be optimized to require only one
SQL query for the queryset evaluation and retrieval of all objects.
Probably all that would be needed seems support for an additional
queryset function in Django's database layer, like::
ModelA.objects.join_models(on="field_name", models=[ModelB, ModelC])
or, less general but more simple::
ModelA.objects.join_tables(on="field_name", tables=['myapp_modelb','myapp_modelc'])
This would add additional left outer joins to the query and then add
the resulting fields from this join to the result objects.
E.g. a query for ``ModelA`` objects would need to join the ``ModelB``
and ``ModelC`` tables on the the field ``id`` and add the fields ``field2``
and ``field3`` from the joined tables to the resulting objects.
An optimization like this might require an SQL database.
For non-SQL databases the implementation could fall back to
the current ORM-only implementation.
SQL Complexity of an Optimized Implementation
---------------------------------------------
With only one SQL query, one SQL join for each possible subclass
would be needed (``BaseModel.__subclasses__()``, recursively).
With two SQL queries, the number of joins could be reduced to the
number of actuallly occurring subclasses in the specific result.
A perfect implementation might want to use one query only
if the number of possible subclasses (and therefore joins) is not
too large, and two queries otherwise (using the first query to
determine the actually occurring subclasses, reducing the number
of joins for the second).
The number of joins needed for polymorphic object retrieval might
raise concerns regarding the efficiency of these database
queries. It seems likely however, that the increased number of joins
is no problem for the supported DBM systems in all realistic use cases.
Should the number of joins of the more extreme use cases turn out to
be problematic, it is possible to split any problematic query into, for example,
two queries with only half the number of joins each.
It seems that further optimization (down to one DB request)
of django_polymorphic would be restricted to a relatively small area of
the code ("query.py"), and be pretty much independent from the rest of the module.
Such an optimization can be done at any later time (like when it's needed).
.. _restrictions:
Restrictions & Caveats
======================
* Queryset methods ``values()``, ``values_list()``, ``select_related()``,
``defer()`` and ``only()`` are not yet fully supported (see above).
``extra()`` has one restriction: the resulting objects are required to have
a unique primary key within the result set
* Django Admin Integration: There currently is no specific admin integration,
but it would most likely make sense to have one.
* Diamond shaped inheritance: There seems to be a general problem
with diamond shaped multiple model inheritance with Django models
(tested with V1.1 - V1.3).
An example is here: http://code.djangoproject.com/ticket/10808.
This problem is aggravated when trying to enhance models.Model
by subclassing it instead of modifying Django core (as we do here
with PolymorphicModel).
* The enhanced filter-definitions/Q-objects only work as arguments
for the methods of the polymorphic querysets. Please see above
for ``translate_polymorphic_Q_object``.
* A reference (``ContentType``) to the real/leaf model is stored
in the base model (the base model directly inheriting from
PolymorphicModel). You need to be aware of this when using the
``dumpdata`` management command or any other low-level
database operations. E.g. if you rename models or apps or copy
objects from one database to another, then Django's ContentType
table needs to be corrected/copied too. This is of course generally
the case for any models using Django's ContentType.
* Django 1.1 only - the names of polymorphic models must be unique
in the whole project, even if they are in two different apps.
This results from a restriction in the Django 1.1 "related_name"
option (fixed in Django 1.2).
* Django 1.1 only - when ContentType is used in models, Django's
seralisation or fixtures cannot be used (all polymorphic models
use ContentType). This issue seems to be resolved for Django 1.2
(changeset 11863: Fixed #7052, Added support for natural keys in serialization).
+ http://code.djangoproject.com/ticket/7052
+ http://stackoverflow.com/questions/853796/problems-with-contenttypes-when-loading-a-fixture-in-django
Project Status
==============
Django_polymorphic works well for a considerable number of users now,
and no major problems have shown up for many months.
The API can be considered stable beginning with the V1.0 release.
Links
=====
- http://code.djangoproject.com/wiki/ModelInheritance
- http://lazypython.blogspot.com/2009/02/second-look-at-inheritance-and.html
- http://www.djangosnippets.org/snippets/1031/
- http://www.djangosnippets.org/snippets/1034/
- http://groups.google.com/group/django-developers/browse_frm/thread/7d40ad373ebfa912/a20fabc661b7035d?lnk=gst&q=model+inheritance+CORBA#a20fabc661b7035d
- http://groups.google.com/group/django-developers/browse_thread/thread/9bc2aaec0796f4e0/0b92971ffc0aa6f8?lnk=gst&q=inheritance#0b92971ffc0aa6f8
- http://groups.google.com/group/django-developers/browse_thread/thread/3947c594100c4adb/d8c0af3dacad412d?lnk=gst&q=inheritance#d8c0af3dacad412d
- http://groups.google.com/group/django-users/browse_thread/thread/52f72cffebb705e/b76c9d8c89a5574f
- http://peterbraden.co.uk/article/django-inheritance
- http://www.hopelessgeek.com/2009/11/25/a-hack-for-multi-table-inheritance-in-django
- http://stackoverflow.com/questions/929029/how-do-i-access-the-child-classes-of-an-object-in-django-without-knowing-the-name/929982#929982
- http://stackoverflow.com/questions/1581024/django-inheritance-how-to-have-one-method-for-all-subclasses
- http://groups.google.com/group/django-users/browse_thread/thread/cbdaf2273781ccab/e676a537d735d9ef?lnk=gst&q=polymorphic#e676a537d735d9ef
- http://groups.google.com/group/django-users/browse_thread/thread/52f72cffebb705e/bc18c18b2e83881e?lnk=gst&q=model+inheritance#bc18c18b2e83881e
- http://code.djangoproject.com/ticket/10808
- http://code.djangoproject.com/ticket/7270
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