Django model order_by based on another model queryset length

Question

I have 2 models:

models.py

class Post(models.Model):
    title = models.CharField(max_length=255)
    desc = models.CharField(max_length=500)
    content = models.TextField()
    uploadedBy = models.ForeignKey(User, on_delete=models.CASCADE)
    
LIKE_CATEGORIES = (
    ("LIKE", "LIKE"),
    ("DISLIKE", "DISLIKE")
)

class PostLike(models.Model):
    _type = models.CharField(max_length=200, choices=LIKE_CATEGORIES, blank=True, null=True)
    
    content = models.ForeignKey(
        Post, on_delete=models.CASCADE)
        
    user = models.ForeignKey(User, on_delete=models.CASCADE)
    
    created = models.DateTimeField(auto_now_add=True)

I want to order Post based on number of likes a post has. Number of likes a post has is the count of PostLike objects having _type="LIKE",content=post_object .

How can I order Post specifically based on number of likes?

Answer 1

You can annotate each Post with aCount of related PostLike objects, this can have a filter to only count the related objects that satisfy a condition. Then order by the annotation

from django.db.models import Count, Q
Post.objects.annotate(
    likes=Count('postlike', filter=Q(postlike___type="LIKE"))
).order_by('-likes')

I'm not sure having a field with an underscore prefix is the best idea, it may mess up the ORM and other parts of the Django internals

Answer 2

This will restrict your filter to Post with LIKE and also rank it

from django.db.models import Count
Post.objects.filter(postlike___type='LIKE').annotate(likes=Count('postlike')).order_by('-likes')

Or

from django.db.models import Count
Post.objects.filter(postlike___type='LIKE').annotate(Count('postlike'))[::-1]