How to count deep relationship fields with Django? - django-models

Question

How could I count the relationship of the models below?
class Client(models.Model):
name = models.CharField(max_length=150)
class Pet(models.Model):
client = models.ForeignKey(Client, related_name='pet')
name = models.CharField(max_length=150)
class Photo(models.Model):
pet = models.ForeignKey(Pet, related_name='photo')
photo = models.ImageField(storage=DIR)
I would like to return the total pet and total photos taken of all pets dr to a single customer client, with the following answer:
[
{
"name": "john",
"totalPet": 3, // imagine 10 photos were taken of each pet
"totalPhoto": 30,
},
{
"name": "Mary",
"totalPet": 2, // imagine that 10 photos were taken of one and 5
// of another pet
"totalPhoto": 15,
},
]
My serializer is as follows:
class ClientSerializer(ModelSerializer):
totalPet = serializers.SerializerMethodField()
def get_totalPet(self, obj):
return obj.pet.count()
class Meta:
model = Client
fields = ('name', 'totalPet')
As can be observed, I can total the amount of pets for each customer, but I can't find a way to total the photos of all pets of a customer.
What would be the best way to do this photo count?

Answer 1

You can perform a filter on Photo model with your client.pet queryset, using the __in lookup.
At first, I would define a custom QuerySet for our Photo model:
from django.db import models
class PhotoQuerySet(models.QuerySet):
def for_client(self, client):
# Get all related pets to the current client
client_pets = client.pet.all()
# Filter Photos that are linked with these pets
return self.filter(pet__in=client_pets)
And then inside the model:
class Photo(models.Model):
...
objects = PhotoQuerySet.as_manager()
This will allow you to do the following: Photo.objects.for_client(client=...)
After that, you can use this method in the serializer:
class ClientSerializer(ModelSerializer):
total_pets = serializers.SerializerMethodField()
total_photos = serializers.SerializerMethodField()
def get_total_pets(self, obj):
return obj.pet.count()
def get_total_photos(self, obj):
# Calling our newly created method and aggregating count
return Photo.objects.for_client(client=obj).count()
class Meta:
model = Client
fields = ('name', 'total_pets', 'total_photos')