Django REST Framework: create and update an object with a nested object value (instead of Primary Key)

Read-only

To simply send that representation to the client (read-only, not dealing with creating objects from their deserialized representation)

class OfficeSerializer(serializers.ModelSerializer):
    country = serializers.Field(source='country.iso') # this field of the serializer
                                                      # is read-only

As you can see, it'll read will read country.iso from your office instance, which resolves to 'us' e.g., then gets put into a serializer key called 'country', given you an output of {'country': 'us'}

Writable nested field

Now to complete this, let's write a custom OfficeSerializer.create():

def create(self, validated_data):
    # This expects an input format of {"country": "iso"}

    # Instead of using the ID/PK of country, we look it up using iso field
    country_iso = validated_data.pop('country')
    country = Country.objects.get(iso=country_iso)

    # Create the new Office object, and attach the country object to it
    office = Office.objects.create(country=country, **validated_data)

    # Notice I've left **validated_data in the Office object builder,
    # just in case you want to send in more fields to the Office model

    # Finally a serializer.create() is expected to return the object instance
    return office

As for a OfficeSerializer.update() it's similar:

def update(self, instance, validated_data):
    # instance is your Office object
    # You should update the Office fields here
    # instance.field_x = validated_data['field_x']

    # Let's grab the Country object again

    country_iso = validated_data.pop('country')
    country = Country.objects.get(iso=country_iso)

    # Update the Office object
    instance.country = country
    instance.save()

    return instance