How to join 3 tables in query with Django, combined select related and prefetch_related - django-models

Question

I have tree models
class Category(models.Model):
name = models.CharField(max_length=60)
class Product(models.Model):
description = = models.CharField(max_length=255)
category = models.ForeignKey(Category, related_name='products')
class Inventory(models.Model):
userReservation = models.ForeignKey(User)
Product=models.ForeignKey(Product related_name='products_reservation')
Quantity = models.IntegerField()
I want to get all product by category, with quantity registered in inventory, for the user and the product
{
"id": 1,
"name": "Corbata",
"products": [{
"id": 10,
"description": "shoes",
"quantitybyInventory": 3
},
{
"id": 9,
"description": "shirt",
"quantitybyInventory": 1
}]}
I have a view with this class
views.py
class inventoryList(generics.ListCreateAPIView):
queryset = Category.objects.prefetch_related('products')
serializer_class = CategorybyProductSerializer
def get_object(self):
queryset = self.queryset()
obj = get_object_or_404(queryset)
return obj
And my serializer
class CategorybyProductSerializer(serializers.ModelSerializer):
products = ProductSerializer(many=True)
class Meta:
model = Category
fields = ('id', 'name', 'products')
class ProductSerializer(serializers.ModelSerializer):
class Meta:
model = Product
fields = ('id', 'description')
but I can not show the amount of the inventory table
this query just show me it
{
"id": 1,
"name": "Corbata",
"products": [{
"id": 10,
"description": "shoes"
},
{
"id": 9,
"description": "shirt"
}
]
}

Answer 1

I use SerializerMethodField, and this post
class ProductbyUserSerializer(serializers.ModelSerializer):
quantitybyInventory = serializers.SerializerMethodField(read_only=True)
def get_quantitybyInventory(self, product):
return product.products_reservation.filter(userReservation = self.context['request'].user).count()
class Meta:
model = Product
fields = ('id', 'description', 'quantitybyInventory')