How to Assign Foreign Keys in Nested Serializers with Django REST Framework?

DRF: Simple Foreign Key Assignment with Nested Serializers

Within Django REST Framework, a standard ModelSerializer permits assignment or modification of ForeignKey model relationships via POSTing an ID as an integer. However, extracting this behavior from a nested serializer can be a challenge.

Simplest Nested Serializer Approach

DRF doesn't inherently provide this functionality, necessitating a custom serializer field:

from rest_framework import serializers

class RelatedFieldAlternative(serializers.PrimaryKeyRelatedField):
    def __init__(self, **kwargs):
        # Serializer for this field
        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):
        # Avoid using PK optimization if a serializer is provided
        return False if self.serializer else True

    def to_representation(self, instance):
        # Use provided serializer or default representation
        if self.serializer:
            return self.serializer(instance, context=self.context).data
        return super().to_representation(instance)

In your parent serializer, employ this new field as follows:

class ParentSerializer(ModelSerializer):
    child = RelatedFieldAlternative(queryset=Child.objects.all(), serializer=ChildSerializer)

    class Meta:
        model = Parent
        fields = '__all__'

With this generic approach, you can readily handle multiple serializers with this functionality.

Method to Override to_representation()

Alternatively, overriding the to_representation() method of the parent serializer also achieves this goal:

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

This method requires defining different fields for creation and reading, which may not be ideal.

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