Django join unmanaged tables - django

Question

I have a four models which each contains their own data. The models are:
Category (contains department_id foreign key)
Department (contains data, no foreign key)
ProductCategory (join table containing only product_id and category_id)
Product (contains data with no foreign key)
# models.py (excluded the rest for brevity)
from django.db import models
class Department(models.Model):
department_id = models.AutoField(primary_key=True)
name = models.CharField(max_length=100)
description = models.CharField(max_length=1000, blank=True, null=True)
class Meta:
managed = False
db_table = 'department'
class Category(models.Model):
category_id = models.AutoField(primary_key=True)
#department_id = models.IntegerField()
department = models.ForeignKey(Department, on_delete=models.CASCADE)
name = models.CharField(max_length=100)
description = models.CharField(max_length=1000, blank=True, null=True)
class Meta:
managed = False
db_table = 'category'
class Product(models.Model):
product_id = models.AutoField(primary_key=True)
name = models.CharField(max_length=100)
description = models.CharField(max_length=1000)
price = models.DecimalField(max_digits=10, decimal_places=2)
discounted_price = models.DecimalField(max_digits=10, decimal_places=2)
image = models.CharField(max_length=150, blank=True, null=True)
image_2 = models.CharField(max_length=150, blank=True, null=True)
thumbnail = models.CharField(max_length=150, blank=True, null=True)
display = models.SmallIntegerField()
class Meta:
managed = False
db_table = 'product'
class ProductCategory(models.Model):
product = models.ForeignKey(Product, on_delete=models.CASCADE)
category = models.ForeignKey(Category, on_delete=models.CASCADE)
class Meta:
managed = False
db_table = 'product_category'
unique_together = (('product', 'category'),)
From my endpoint, I need to get all products in a department and return the response in the following format:
​"rows"​: [
{
​ "product_id"​: ​integer​,
"name"​: ​string​,
​"description"​: ​string​,
"price"​: ​string​,
​"discounted_price"​: ​string​,
"thumbnail"​: ​string​
}
]
This is the endpoint:
path('products/inDepartment/<int:department_id>/', ProductViewSet.as_view({"get": "get_products_by_department"}))
How can I go about doing this? I'm stuck with the code below:
# products.py
def get_products_by_department(self, request, department_id):
"""
Get a list of Products of Departments
"""
categories = Category.objects.filter(department_id=department_id).values('category_id')
for item in categories:
category_id = item['category_id']
products = ProductCategory.objects.filter(category_id=category_id).values(
'product_id', name=F('product__name'), description=F('product__description'),
price=F('product__price'), discounted_price=F('product__discounted_price'), thumbnail=F('product__thumbnail'))
# Return response
if products.exists():
return Response(products, 200)
else:
return Response(products, 204)
The code above works and gives me the correct response but I'm not sure if I'm doing the query correctly? Should I be using a loop or is there a Django way to do it better without a loop?

Answer 1

Django's ORM allows for reverse relationship lookup.
https://docs.djangoproject.com/en/2.2/topics/db/queries/#lookups-that-span-relationships
categories = Category.objects.filter(department_id=department_id)
products = Product.objects.filter(productcategory__category__in=categories)