Skip to content
Back to top
View this page
Edit this page

# Advanced Features

In the examples below, these models are being used:

    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 `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 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`):

    >>> 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:

    >>> 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):

        # 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 `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 `save()`.

The Django documentation offers some discussion on copying, including the complexity around related fields and multi-table inheritance. django-polymorphic offers a utility function `prepare_for_copy()` that resets all necessary fields on a model instance to prepare it for copying:

    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 `dumpdata` and `loaddata` commands just as regular models do. There are two important considerations:

1. Polymorphic models are multi-table models and `dumpdata` serializes each table separately. django-polymorphic does it’s best to ensure non-polymorphic managers are used when creating fixtures but there may be edge cases where this fails. If you override `dumpdata` you must make sure any polymorphic managers encountered `toggle polymorphism off`. 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 `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 `--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 `--natural-primary` and `--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 `loaddata`. However, if there are problems with the `polymorphic_ctype` references, you may fix them using `reset_polymorphic_ctype()`:

    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:

    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:

    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()` 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 inherited 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 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()` 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. `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:

    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 `ShowFieldType` class mixin:

    from polymorphic.models import PolymorphicModel
    from polymorphic.showfields import 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 behavior 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`.

## Create Children from Parents (Downcasting)

You can create an instance of a subclass from an existing instance of a superclass using the `create_from_super()` method of the subclass’s manager. For example:

    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 `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 `delete()`. django-polymorphic ensures that the `polymorphic_ctype` fields of the superclass instances are updated accordingly when doing this.

## Restrictions & Caveats

- Database Performance regarding concrete Model inheritance in general. Please see Performance Considerations.

- Queryset methods `values()`, `values_list()`, and `select_related()` 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.

- 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. This problem is aggravated when trying to enhance `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`.

- When using the `dumpdata` management command on polymorphic tables (or any table that has a reference to `ContentType`), include the `--natural-primary` and `--natural-foreign` flags in the arguments.

- If the `polymorphic_ctype_id` on the base table points to the wrong `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, `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.