How to Simplify Foreign Key Assignment in Nested Django REST Framework Serializers?

DRF: Simplified Foreign Key Assignment in Nested Serializers

Problem:

With Django REST Framework (DRF), a standard ModelSerializer allows ForeignKey model relationships to be assigned or edited by posting an ID as an integer. However, when working with nested serializers, replicating this behavior raises doubts on the best approach.

Solution:

Overriding to_representation() Method

One method to achieve this functionality in a nested serializer is to override the to_representation() method in the parent serializer. This technique has the following advantages:

• No need for separate fields for creation and reading.
• Both creation and reading can be done using the same key.

Sample Parent Serializer with Modified to_representation() Method:

class ParentSerializer(ModelSerializer):

    class Meta:
        model = Parent
        fields = '__all__'

    def to_representation(self, instance):
        response = super().to_representation(instance)
        response['child'] = ChildSerializer(instance.child).data
        return response

Using a Custom Serializer Field

For a more generic solution, consider creating a custom serializer field called RelatedFieldAlternative. This field ensures compatibility with DRF versions 3.x and 4.x.

Custom Serializer Field:

from rest_framework import serializers

class RelatedFieldAlternative(serializers.PrimaryKeyRelatedField):

    def __init__(self, **kwargs):
        self.serializer = kwargs.pop('serializer', None)
        if self.serializer is not None and not issubclass(self.serializer, serializers.Serializer):
            raise TypeError('"serializer" is not a valid serializer class')

        super().__init__(**kwargs)

    def use_pk_only_optimization(self):
        return False if self.serializer else True

    def to_representation(self, instance):
        if self.serializer:
            return self.serializer(instance, context=self.context).data
        return super().to_representation(instance)

Using the Custom Field in the Parent Serializer:

class ParentSerializer(ModelSerializer):

    child = RelatedFieldAlternative(queryset=Child.objects.all(), serializer=ChildSerializer)

    class Meta:
        model = Parent
        fields = '__all__'

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