Querying Reverse ManyToMany Relationship in Django - django

Question

I have a category Model and a Product Model in my django application... the product model has a many2many field pointing to the category model... and the product model has a added field to denote when it was created.
how do i filter the categories model based on the added field value of the product?
class Category(models.Model):
#this model SHOULD BE managed by the platform admin
title = models.CharField(max_length=100)
slug = models.SlugField(blank=True)
description = models.CharField(max_length=255, blank=True, null=True)
items_count = models.PositiveIntegerField(null=True, blank=True)
updated = models.DateTimeField(auto_now=True)
image = models.ImageField(upload_to='categories_images',help_text='Generic Image describing the category very very well', null=True, blank=True)
def __str__(self):
return str(self.slug)
def save(self, *args, **kwargs):
self.slug = slugify(self.title)
super().save(*args, **kwargs)
class Product(models.Model):
title = models.CharField(max_length=50)
image = models.ImageField(upload_to='uploaded_products/', default="products/default_product.png")
description = models.TextField(max_length=255, null=True, blank=True)
categories = models.ManyToManyField(Category,related_name='products', blank=True)
tags = models.ManyToManyField(Tag, blank=True)
stock = models.PositiveIntegerField(default=1, blank=True)
added = models.DateTimeField(auto_now_add=True)
updated = models.DateTimeField(auto_now=True)
uploader = models.ForeignKey(Account, on_delete=models.CASCADE, blank=True, null=True)
def __str__(self):
return self.title
i want to be able to do a query for the current trending categories based on the added field of the product.

Answer 1

We can first look for each Category when was the most recent Product that was added with .alias(…) [Django-doc], or with .annotate(…) [Django-doc] prior to django-3.2. We annotate this with:
from django.db.models import Max
Category.objects.alias(
most_recent_change=Max('products__added')
)
Then we can retrieve the objects through filtering. For example if we only want to retrieve Categorys updated in the last 24 hours, we can filter with:
from datetime import timedelta
from django.db.models import Max
from django.db.models.functions import Now
Category.objects.alias(
most_recent_change=Max('products__added')
).filter(
most_recent__gte=Now()-timedelta(days=1)
)
or we can make use of the timestamp provided by Django itself:
from datetime import timedelta
from django.utils.timezone import now
from django.db.models import Max
Category.objects.alias(
most_recent_change=Max('products__added')
).filter(
most_recent__gte=now()-timedelta(days=1)
)