Django REST Framework: make a serializer behave different for request payload vs response body?

Question

I'm trying to come up with an easy way to make my serializers and views work the way I want to, while preventing a bunch of manual boilerplate.

My simplified model:

class Character(models.Model):
    name = models.CharField(max_length=255)
    location = models.ForeignKey(Location, on_delete=models.SET_NULL, blank=True, null=True)
    factions = models.ManyToManyField(Faction, blank=True)

So: character can have one location, and multiple factions.

Now, when I GET a character, I'd like those submodels (Location and Faction) to be expanded to their full representation, so my serializer looks like this:

class CharacterSerializer(serializers.ModelSerializer):
    location = LocationSerializer(read_only=True)
    factions = FactionSerializer(many=True, read_only=True)

    class Meta:
        model = Character
        fields = "__all__"

And this works perfectly fine so far. The thing is, when I POST or PUT a character, I'd like to just send the id(s).. but still get the full location and faction objects back in the response.

In other words, when I create a character with a payload like this:

{"name":"Saga","location":1,"factions":[1]}

I'd like the response to look like this:

{
  "id": 1,
  "location": {
    "id": 1,
    "name": "Location 1"
  },
  "factions": [
    {
      "id": 1,
      "name": "Faction 1"
    }
  ],
  "name": "Saga"
}

Is this possible at all without overriding the create and update methods of my ModelViewSet subclass? I was hoping I could slightly modify the serializer itself to only apply those location and factions field serializers on the response, not on the request.

Answer 1

You can override the create and update methods of the serializer to work with primary keys.

This thus means that we first pop the data from the validated_data and then work with the remaining data to create/update an object. We can then update the many-to-many relation by using .set(…) and .add(…):

class CharacterSerializer(serializers.ModelSerializer):
    location = LocationSerializer(read_only=True)
    factions = FactionSerializer(many=True, read_only=True)

    def create(self, validated_data):
        location_data = validated_data.pop('location')
        factions_data = validated_data.pop('factions')
        character = Character.objects.create(
            **validated_data,
            location_id=location_data
        )
        character.factions.add(*factions_data)

    def update(self, instance, validated_data):
        location_data = validated_data.pop('location')
        factions_data = validated_data.pop('factions')
        instance.location_id = location_data
        result = super().update(instance, validated_data)
        instance.factions.set(faction_data)
        return result

    class Meta:
        model = Character
        fields = '__all__'