How to save foreign key field in django rest framework?

Question

I want to create an order, I'm trying to save a milk order by a user. Basically, the user must choose what type of milk he wants when placing an order. What should I do from my point of view? I'm a little confused. The following code does not give me anything, the data is not saved. What should I do, can someone help?

Here are my files. models.py

MILK_CHOICES = (
('C', 'COW'),
('B', 'BUFFALO'),
('FC', 'FULL CREAM')
)

class MilkType(models.Model):
    type = models.CharField(max_length = 20, choices = MILK_CHOICES, default = MILK_CHOICES[0][1])
    quantity = models.PositiveIntegerField() # Stock
    description = models.CharField(max_length = 100)

class Order(models.Model):
    user = models.ForeignKey(settings.AUTH_USER_MODEL)
    milk_choice = models.ForeignKey(MilkType)
    qty = models.PositiveIntegerField(default = 1)

serializers.py

class UserSerializer(serializers.ModelSerializer):
    orders = serializers.PrimaryKeyRelatedField(many=True, queryset=Order.objects.all())

    class Meta:
        model = CustomUser
        fields = ('id', 'username', 'email')

class MilkTypeSerializer(serializers.ModelSerializer):

    class Meta:
        model = MilkType
        fields = ('id', 'type', 'quantity', 'description')

class OrderSerializer(serializers.ModelSerializer):
    user = serializers.ReadOnlyField(source = 'user.username')

    class Meta:
        model = Order
        fields = ('id', 'user', 'milk_choice', 'qty')

views.py

class OrderList(APIView):

    def get(self, request, format = None):
        orders = Order.objects.all()
        serializer = OrderSerializer(orders, many = True)
        return Response(serializer.data)

    def post(self, request, format = None):
        serializer = OrderSerializer(data = request.data)

        if serializer.is_valid():
            serializer.save(user = request.user)
            return Response(serializer.data, status = status.HTTP_201_CREATED)
        return Response(serializer.errors, status = status.HTTP_400_BAD_REQUEST)

Answer 1

In your OrderSerializer, define the serializer for the field milk_choiceas MilkTypeSerializer.

class OrderSerializer(serializers.ModelSerializer):
    user = serializers.ReadOnlyField(source = 'user.username')
    milk_choice = MilkTypeSerializer()

    class Meta:
        model = Order
        fields = ('id', 'user', 'milk_choice', 'qty')