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django-polymorphic/docs/advanced.rst
Brian Kohan 46b2cacbc6
issued doc corrections based on #814
2026-01-09 12:46:07 -08:00

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.. _advanced-features:
Advanced Features
=================
In the examples below, these models are being used:
.. code-block:: python
from django.db import models
from polymorphic.models 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 :func:`isinstance`):
------------------------------------------------------------------
.. code-block:: python
>>> 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:
.. code-block:: python
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):
.. code-block:: python
>>> ModelA.objects.filter( Q(instance_of=ModelB) )
Polymorphic filtering (for fields in inherited classes)
-------------------------------------------------------
For example, cherry-picking objects from multiple derived classes anywhere in the inheritance tree,
using Q objects (with the syntax: ``exact model name + three _ + field name``):
.. code-block:: python
>>> 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 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:
.. code-block:: python
>>> 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:
.. code-block:: python
field1 = OneToOneField(ModelA)
then field1 may now also refer to objects of type ``ModelB`` or ``ModelC``.
A :class:`~django.db.models.ManyToManyField` example:
.. code-block:: python
# The model holding the relation may be any kind of model, polymorphic or not
class RelatingModel(models.Model):
# ManyToMany relation to a polymorphic model
many2many = models.ManyToManyField('ModelA')
>>> 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)> ]
Copying Polymorphic objects
---------------------------
**Copying polymorphic models is no different than copying regular multi-table models.** You have
two options:
1. Use :meth:`~django.db.models.query.QuerySet.create` and provide all field values from the
original instance except the primary key(s).
2. Set the primary key attribute, and parent table pointers at all levels of inheritance to ``None``
and call :meth:`~django.db.models.Model.save`.
The Django documentation :ref:`offers some discussion on copying <topics/db/queries:copying model instances>`,
including the complexity around related fields and multi-table inheritance.
:pypi:`django-polymorphic` offers a utility function :func:`~polymorphic.utils.prepare_for_copy`
that resets all necessary fields on a model instance to prepare it for copying:
.. code-block:: python
from polymorphic.utils import prepare_for_copy
obj = ModelB.objects.first()
prepare_for_copy(obj)
obj.save()
# obj is now a copy of the original ModelB instance
Working with Fixtures
---------------------
Polymorphic models work with Django's :django-admin:`dumpdata` and :django-admin:`loaddata`
commands just as regular models do. There are two important considerations:
1. Polymorphic models are multi-table models and :django-admin:`dumpdata` serializes each table
separately. :pypi:`django-polymorphic` `does it's best
<https://github.com/jazzband/django-polymorphic/pull/814>`_ to ensure non-polymorphic managers
are used when creating fixtures but there may be edge cases where this fails. If you override
:django-admin:`dumpdata` you must make sure any polymorphic managers encountered
:meth:`toggle polymorphism off <polymorphic.managers.PolymorphicQuerySet.non_polymorphic>`. Other
usual multi-table model caveats apply. If you serialize a subset of tables in the model
inheritance you may generate corrupt data or "upcast" your models if child tables were omitted.
2. Polymorphic models rely on the :class:`~django.contrib.contenttypes.models.ContentType`
framework. When serializing and deserializing polymorphic models, the
``polymorphic_ctype`` field must be handled correctly. If there is any question about if the
content type primary keys are or will be different between the source and target database you
should use the :option:`--natural-foreign <dumpdata.--natural-foreign>` flag to serialize those
relations by-value. Polymorphism introduces no special consideration here - any model using
contenttypes, polymorphic or not, must handle this correctly.
.. note::
Prior documentation urged users to use both :option:`--natural-primary <dumpdata.--natural-primary>`
and :option:`--natural-foreign <dumpdata.--natural-foreign>` flags when dumping polymorphic
models. This is not necessary and only needs to be done when the primary keys are not guaranteed
to match or be available at the target database.
Loading Fixtures (loaddata)
~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Fixtures should be loadable as normal with :django-admin:`loaddata`. However, if there are problems
with the ``polymorphic_ctype`` references, you may fix them using
:func:`~polymorphic.utils.reset_polymorphic_ctype`:
.. code-block:: python
from polymorphic.utils import reset_polymorphic_ctype
from myapp.models import Animal, Dog, Cat
# Reset polymorphic_ctype for all models in the inheritance tree
reset_polymorphic_ctype(Animal, Dog, Cat)
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:
.. code-block:: python
from thirdparty import ThirdPartyModel
class MyThirdPartyBaseModel(PolymorphicModel, ThirdPartyModel):
pass # or add fields
Or instead integrating the third party model anywhere into an
existing polymorphic inheritance tree:
.. code-block:: python
class MyBaseModel(SomePolymorphicModel):
my_field = models.CharField(max_length=10)
class MyModelWithThirdParty(MyBaseModel, ThirdPartyModel):
pass # or add fields
Non-Polymorphic Queries
-----------------------
If you insert :meth:`~polymorphic.managers.PolymorphicQuerySet.non_polymorphic` anywhere into the
query chain, then :pypi:`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).
.. code-block:: python
>>> 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:
.. code-block:: python
>>> 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
----------------------
* :meth:`~django.db.models.query.QuerySet.annotate` and
:meth:`~django.db.models.query.QuerySet.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).
* :meth:`~django.db.models.query.QuerySet.order_by` similarly supports the ``ModelX___field``
syntax for specifying ordering through a field in a submodel.
* :meth:`~django.db.models.query.QuerySet.distinct` works as expected. It only regards the fields
of the base class, but this should never make a difference.
* :meth:`~django.db.models.query.QuerySet.select_related` works just as usual, but it can not
(yet) be used to select relations in inherited models (like
``ModelA.objects.select_related('ModelC___fieldxy')`` )
* :meth:`~django.db.models.query.QuerySet.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 by using
``ModelA.objects.non_polymorphic().extra(...)``.
* :meth:`~polymorphic.managers.PolymorphicQuerySet.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)``
* :meth:`~django.db.models.query.QuerySet.values` &
:meth:`~django.db.models.query.QuerySet.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.
* :meth:`~django.db.models.query.QuerySet.defer` and :meth:`~django.db.models.query.QuerySet.only`
work as expected. On Django 1.5+ they support the ``ModelX___field`` syntax, but on Django 1.4
it is only possible to pass fields on the base model into these methods.
Using enhanced Q-objects in any Places
--------------------------------------
The queryset enhancements (e.g. :meth:`~polymorphic.managers.PolymorphicQuerySet.instance_of`)
only work as arguments to the member functions of a polymorphic queryset. Occasionally it may
be useful to be able to use Q objects with these enhancements in other places. As Django doesn't
understand these enhanced Q objects, you need to transform them manually into normal Q objects
before you can feed them to a Django queryset or function:
.. code-block:: python
normal_q_object = ModelA.translate_polymorphic_Q_object( Q(instance_of=Model2B) )
This function cannot be used at model creation time however (in models.py), as it may need to access
the ContentTypes database table.
Nicely Displaying Polymorphic Querysets
---------------------------------------
In order to get the output as seen in all examples here, you need to use the
:class:`~polymorphic.showfields.ShowFieldType` class mixin:
.. code-block:: python
from polymorphic.models import PolymorphicModel
from polymorphic.showfields import ShowFieldType
class ModelA(ShowFieldType, PolymorphicModel):
field1 = models.CharField(max_length=10)
You may also use :class:`~polymorphic.showfields.ShowFieldContent` or
:class:`~polymorphic.showfields.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 behavior
by setting a class variable in your model like this:
.. code-block:: python
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``.
Create Children from Parents (Downcasting)
------------------------------------------
You can create an instance of a subclass from an existing instance of a superclass using the
:meth:`~polymorphic.managers.PolymorphicManager.create_from_super` method
of the subclass's manager. For example:
.. code-block:: python
super_instance = ModelA.objects.get(id=1)
sub_instance = ModelB.objects.create_from_super(super_instance, field2='value2')
The restriction is that ``super_instance`` must be an instance of the direct superclass of
``ModelB``, and any required fields of ``ModelB`` must be provided as keyword arguments. If multiple
levels of subclassing are involved, you must call this method multiple times to "promote" each
level.
Delete Children, Leaving Parents (Upcasting)
--------------------------------------------
The reverse operation of :meth:`~polymorphic.managers.PolymorphicManager.create_from_super` is to
delete the subclass instance while keeping the superclass instance. This can be done using the
``keep_parents=True`` argument to :meth:`~django.db.models.Model.delete`. :pypi:`django-polymorphic`
ensures that the ``polymorphic_ctype`` fields of the superclass instances are updated accordingly
when doing this.
.. _restrictions:
Restrictions & Caveats
----------------------
* Database Performance regarding concrete Model inheritance in general. Please see
:ref:`performance`.
* Queryset methods :meth:`~django.db.models.query.QuerySet.values`,
:meth:`~django.db.models.query.QuerySet.values_list`, and
:meth:`~django.db.models.query.QuerySet.select_related` are not yet fully supported (see above).
:meth:`~django.db.models.query.QuerySet.extra` has one restriction: the resulting objects are
required to have a unique primary key within the result set.
* 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 :class:`~django.db.models.Model` by subclassing it instead of modifying Django core
(as we do here with :class:`~polymorphic.models.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``.
* When using the :django-admin:`dumpdata` management command on polymorphic tables
(or any table that has a reference to :class:`~django.contrib.contenttypes.models.ContentType`),
include the :option:`--natural-primary <dumpdata.--natural-primary>` and
:option:`--natural-foreign <dumpdata.--natural-foreign>` flags in the arguments.
* If the ``polymorphic_ctype_id`` on the base table points to the wrong
:class:`~django.contrib.contenttypes.models.ContentType` (this can happen if you delete child
rows manually with raw SQL, ``DELETE FROM table``), then polymorphic queries will elide the
corresponding model objects:
* ``BaseClass.objects.all()`` will **exclude** these rows (it filters for existing child types).
* ``BaseClass.objects.non_polymorphic().all()`` will behave as normal - but polymorphic
behavior for the affected rows will be undefined - for instance,
:meth:`~polymorphic.managers.PolymorphicQuerySet.get_real_instances` will raise an
exception.
Always use ``instance.delete()`` or ``QuerySet.delete()`` to ensure cascading deletion of the
base row. If you must delete manually, ensure you also delete the corresponding row from the
base table.
.. old 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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