DRF serializer for instance creation with related keys and response with nested serializer representation

How to create a serializer which accepts keys of related instances for input (creation & update) and apply a nested serializer for the output

I have 2 models in my application: Order and Item. They are related with a ManyToMany field.

# models.py

class Order(models.Model):
    referrence = models.CharField(max_length=50)
    items = models.ManyToManyField(to=Item, related_name='order')

class Item(models.Model):
    name = models.CharField(max_length=50)
    price = models.IntegerField()

I have an endpoint with a view which is using the concrete view class CreateAPIView to create a new order.

# views.py

class ListCreateOrdersView(CreateAPIView):
    serializer_class = OrderSerializer

To create a new order with the related recipes already set, I could send a request with a list of recipe id's. With following serializer, that would work. However, I would also only get the id's in the response.

# serializers.py

class OrderSerializer(serializers.ModelSerializer):
    class Meta:
        model = Order
        fields = ['id', 'referrence', 'items']

request body

{
  "reference": "Christmas party",
  "items": [2, 3]
}

response body

{
  "id": "1",
  "reference": "Christmas party",
  "items": [2, 3]  
}

My goal is to be able to send the item id's in the request body, but to get more details about each item in the response.

The most elegant way I found is by overwriting the to_representation method and adding the nested serializer there. With to_representation() the serialization output can be modified.

# serializers.py
class ItemSerializer(serializers.ModelSerializer):
    class Meta:
        model = Item
        fields = ['id', 'name', 'price']

class OrderSerializer(serializers.ModelSerializer):
    class Meta:
        model = Order
        fields = ['id', 'referrence', 'items']

    def to_representation(self, instance):
        representation = super().to_representation(instance)
        # At this step, representation['items'] is a list of item id's. In the next step, I use the ItemSerializer to
        # overwrite that list with serialized data for each instance.
        representation['items'] = ItemSerializer(instance.items, many=True).data
        return representation

Now, I still send the same request but get following response.

request body

{
  "reference": "Christmas party",
  "items": [2, 3]
}

response body

{
  "id": "1",
  "reference": "Christmas party",
  "items": [
    {
      "id": 2,
      "name": "wine",
      "price": 15
    }, 
    {
      "id": 3,
      "name": "cheese",
      "price": 10
    }
  ]  
}