Writable nested serializer with 'source' argument

[pwellever]

I am encountering an issue implementing writable serializers with nested relationships. Below is a simplified example:

class Parent(models.Model):
    name = models.CharField(max_length=255)
    age = models.PositiveIntegerField()


class Child(models.Model):
    name = models.CharField(max_length=255)
    age = models.PositiveIntegerField()
    parent = models.ForeignKey('Parent',
                               blank=True,
                               null=True,
                               related_name='children',
                               on_delete=models.SET_NULL)

class ChildSerializer(serializers.ModelSerializer):
    class Meta:
        model = Child
        fields = ['name', 'age']


class ParentSerializer(serializers.ModelSerializer):
    offspring = ChildSerializer(many=True, source='children', required=False)

    class Meta:
        model = Parent
        fields = ['name', 'age', 'offspring']

    def create(self, validated_data):
        offspring = validated_data.pop('offspring')
        instance = super().create(validated_data)
        for item in offspring:
            instance.children.create(**item)
        return instance

Notice that on ParentSerializer, the field name is offspring, whereas the name of the reverse relation on the Parent model is children. The serializer field uses the source= argument to address this. And indeed, ParentSerializer behaves as expected when used to display existing data.

However, when attempting to save a new instance...

>>> ps = ParentSerializer(data={'name': 'John', 'age': 37, 'offspring': [{'name': 'Tommy', 'age': 3}]})
>>> ps.is_valid()
True
>>> ps.validated_data
OrderedDict([('name', 'John'), ('age', 37), ('children', [OrderedDict([('name', 'Tommy'), ('age', 3)])])])

The validated_data dict has altered the field name from offspring to children. Shouldn't this be respecting the field name declared on the serializer instead of conforming it to the model definition? This behavior does not seem to happen with "normal" serializer fields that use the source argument -- it's only appearing when the serializer field is another (nested) serializer.

Is this the intended behavior?