tl;dr: It isn't as complicated as it looks. This solution proposes a fully reusable class that realizes the requested usecase with a minimum of code, as one can see from the exemplary usage below.
After some fiddling, I've come up with following solution, that, I believe, is quite satisfying. We will need one helper function and two classes for this, there are no further dependencies.
An extension to theHyperlinkedModelSerializer
Suppose that a query returned an object from Animal class, that actually is a Bird. Than get_actual would resolve that Animal to an object form Bird class:
def get_actual(obj):
"""Expands `obj` to the actual object type.
"""
for name in dir(obj):
try:
attr = getattr(obj, name)
if isinstance(attr, obj.__class__):
return attr
except:
pass
return obj
The ModelField defines a field that names the model that underlies a serializer:
class ModelField(serializers.ChoiceField):
"""Defines field that names the model that underlies a serializer.
"""
def __init__(self, *args, **kwargs):
super(ModelField, self).__init__(*args, allow_null=True, **kwargs)
def get_attribute(self, obj):
return get_actual(obj)
def to_representation(self, obj):
return obj.__class__.__name__
The HyperlinkedModelHierarchySerializer does the magic:
class HyperlinkedModelHierarchySerializer(serializers.HyperlinkedModelSerializer):
"""Extends the `HyperlinkedModelSerializer` to properly handle class hierearchies.
For an hypothetical model `BaseModel`, serializers from this
class are capable of also handling those models that are derived
from `BaseModel`.
The `Meta` class must whitelist the derived `models` to be
allowed. It also must declare the `model_dependent_fields`
attribute and those fields must also be added to its `fields`
attribute, for example:
wing = serializers.CharField(allow_null=True)
tail = serializers.CharField(allow_null=True)
class Meta:
model = Animal
models = (Bird, Dog)
model_dependent_fields = ('wing', 'tail')
fields = ('model', 'id', 'name') + model_dependent_fields
read_only_fields = ('id',)
The `model` field is defined by this class.
"""
model = ModelField(choices=[])
def __init__(self, *args, **kwargs):
"""Instantiates and filters fields.
Keeps all fields if this serializer is processing a CREATE
request. Retains only those fields that are independent of
the particular model implementation otherwise.
"""
super(HyperlinkedModelHierarchySerializer, self).__init__(*args, **kwargs)
# complete the meta data
self.Meta.models_by_name = {model.__name__: model for model in self.Meta.models}
self.Meta.model_names = self.Meta.models_by_name.keys()
# update valid model choices,
# mark the model as writable if this is a CREATE request
self.fields['model'] = ModelField(choices=self.Meta.model_names, read_only=bool(self.instance))
def remove_missing_fields(obj):
# drop those fields model-dependent fields that `obj` misses
unused_field_keys = set()
for field_key in self.Meta.model_dependent_fields:
if not hasattr(obj, field_key):
unused_field_keys |= {field_key}
for unused_field_key in unused_field_keys:
self.fields.pop(unused_field_key)
if not self.instance is None:
# processing an UPDATE, LIST, RETRIEVE or DELETE request
if not isinstance(self.instance, QuerySet):
# this is an UPDATE, RETRIEVE or DELETE request,
# retain only those fields that are present on the processed instance
self.instance = get_actual(self.instance)
remove_missing_fields(self.instance)
else:
# this is a LIST request, retain only those fields
# that are independent of the particular model implementation
for field_key in self.Meta.model_dependent_fields:
self.fields.pop(field_key)
def validate_model(self, value):
"""Validates the `model` field.
"""
if self.instance is None:
# validate for CREATE
if value not in self.Meta.model_names:
raise serializers.ValidationError('Must be one of: ' + (', '.join(self.Meta.model_names)))
else:
return value
else:
# model cannot be changed
return get_actual(self.instance).__class__.__name__
def create(self, validated_data):
"""Creates instance w.r.t. the value of the `model` field.
"""
model = self.Meta.models_by_name[validated_data.pop('model')]
for field_key in self.Meta.model_dependent_fields:
if not field_key in model._meta.get_all_field_names():
validated_data.pop(field_key)
self.fields.pop(field_key)
return model.objects.create(**validated_data)
Exemplary Usage
And this is how we can use it. In serializers.py:
class AnimalSerializer(HyperlinkedModelHierarchySerializer):
wing = serializers.CharField(allow_null=True)
tail = serializers.CharField(allow_null=True)
class Meta:
model = Animal
models = (Bird, Dog)
model_dependent_fields = ('wing', 'tail')
fields = ('model', 'id', 'name') + model_dependent_fields
read_only_fields = ('id',)
And in views.py:
class AnimalViewSet(viewsets.ModelViewSet):
queryset = Animal.objects.all()
serializer_class = AnimalSerializer
