Django REST framework: save related models in ModelViewSet

I'm trying to figure out how to save related models using Django REST framework. In my app I have a model Recipe with 2 related models: RecipeIngredient and RecipeStep. A Recipe object MUST have at least 3 related RecipeIngredient and 3 RecipeStep. Before the introduction of the REST framework I was using a Django CreateView with two formsets and the save process was the following (follow the code from form_valid()):

def save_formsets(self, recipe):
    for f in self.get_formsets():
        f.instance = recipe
        f.save()

def save(self, form):
    with transaction.atomic():
        recipe = form.save()
        self.save_formsets(recipe)
    return recipe

def formsets_are_valid(self):
        return all(f.is_valid() for f in self.get_formsets())

def form_valid(self, form):
    try:
        if self.formsets_are_valid():
            try:
                return self.create_ajax_success_response(form)
            except IntegrityError as ie:
                return self.create_ajax_error_response(form, {'IntegrityError': ie.message})
    except ValidationError as ve:
        return self.create_ajax_error_response(form, {'ValidationError': ve.message})
    return self.create_ajax_error_response(form)

Now I have my RecipeViewSet:

class RecipeViewSet(ModelViewSet):
    serializer_class = RecipeSerializer
    queryset = Recipe.objects.all()
    permission_classes = (RecipeModelPermission, )

which uses RecipeSerializer:

class RecipeSerializer(serializers.ModelSerializer):
    class Meta:
        model = Recipe
        fields = (
            'name', 'dish_type', 'cooking_time', 'steps', 'ingredients'
        )

    ingredients = RecipeIngredientSerializer(many=True)
    steps = RecipeStepSerializer(many=True)

and these are the related serializers:

class RecipeIngredientSerializer(serializers.ModelSerializer):
    class Meta:
        model = RecipeIngredient
        fields = ('name', 'quantity', 'unit_of_measure')

class RecipeStepSerializer(serializers.ModelSerializer):
    class Meta:
        model = RecipeStep
        fields = ('description', 'photo')

Now... how I'm supposed to validate related models (RecipeIngredient and RecipeStep) and save them when RecipeViewSet's create() method is called? (is_valid() in RecipeSerializer is actually ignoring nested relationships and reporting only errors related to the main model Recipe). At the moment I tried to override the is_valid() method in RecipeSerializer, but is not so simple... any idea?

I was dealing with similiar issue this week and I found out, that django rest framework 3 actually supports nested writable serialisation (http://www.django-rest-framework.org/topics/3.0-announcement/#serializers in subchapter Writable nested serialization.)

Im not sure if nested serialisers are writable be default, so I declared them:

ingredients = RecipeIngredientSerializer(many=True, read_only=False)
steps = RecipeStepSerializer(many=True, read_only=False)

and you should rewrite your create methon inside RecipeSerializer:

class RecipeSerializer(serializers.ModelSerializer):
    ingredients = RecipeIngredientSerializer(many=True, read_only=False)
    steps = RecipeStepSerializer(many=True, read_only=False)

    class Meta:
        model = Recipe
        fields = (
            'name', 'dish_type', 'cooking_time', 'steps', 'ingredients'
        )

    def create(self, validated_data):
        ingredients_data = validated_data.pop('ingredients')
        steps_data = validated_data.pop('steps')
        recipe = Recipe.objects.create(**validated_data)
        for ingredient in ingredients_data:
            #any ingredient logic here
            Ingredient.objects.create(recipe=recipe, **ingredient)
        for step in steps_data:
            #any step logic here
            Step.objects.create(recipe=recipe, **step)
        return recipe

if this structure Step.objects.create(recipe=recipe, **step) wont work, maybe you have to select data representeng each field separatly from steps_data / ingredients_data.

This is link to my earlier (realted) question/answer on stack: How to create multiple objects (related) with one request in DRF?