Django Rest Framework: Count Number of Objects with multiple ManyToManyField values

Question

In the example below, I have two models with a ManyToMany relation and I'm trying to count the number of posts related to the tags.

There are 3 tags: Sports, Films, Health
There are 3 posts: 1 for Sports, 1 for Films and 1 post with two tags (Sports and Health)

I can get the count of posts for every tag as below:

[
    {
        "name": "Sports",
        "posts": 2
    },
    {
        "name": "Films",
        "posts": 1
    },
    {
        "name": "Health",
        "posts": 1
    }

]

My requirement is to count the objects separately for the combination of tags. So, the desirable output is:

[
    {
        "name": "Sports",
        "posts": 1
    },
    {
        "name": "Films",
        "posts": 1
    },
    {
        "name": "Sports & Health",
        "posts": 1
    }

]

This is where I'm stuck. How do I combine the two tags and then count the number of post objects that have both these tags?

models.py

class Tag(models.Model):
    name = models.CharField(max_length=100)

    def __str__(self):
        return self.name


class Post(models.Model):
    title = models.CharField(max_length=100)
    content = models.TextField()
    tags = models.ManyToManyField(Tag)

    def __str__(self):
        return self.title

serializers.py

class TagSerializer(serializers.ModelSerializer):

    posts = serializers.SerializerMethodField()

    class Meta:
        model = Tag
        fields = ('name', 'posts')

    def get_posts(self, obj):
        posts = obj.post_set.all().count()
        return posts


class PostSerializer(serializers.ModelSerializer):
    class Meta:
        model = Post
        fields = '__all__'

Your codes looks correct. Are you sure you have 1 post record for sport tag? — kamilyrb
– kamilyrb, Commented Feb 20, 2020 at 6:48
Yes, I have made 3 posts. One is for Sports only and one other has two tags: Sports and Health. So it is counted as 2 for sports. What i want is to count it separately with combined category (Sports and Health) — Abhijit Raj
– Abhijit Raj, Commented Feb 20, 2020 at 7:00
it is possible to add multiple tags to a post. Which might be happened here — ruddra
– ruddra, Commented Feb 20, 2020 at 7:52
@ruddra: Yes, the default behavior is okay. I want the modification as described in the question which is where I'm stuck. — Abhijit Raj
– Abhijit Raj, Commented Feb 20, 2020 at 10:02

ameen alakhras · Accepted Answer · 2021-07-14 08:49:42Z

2

I would recommend adding a related field in the Post model to the ManyToMany relation ease of calling the many to many relation from the tags side.

tags = models.ManyToManyField(Tag, related_name="posts_objects")

Instead of adding SerializerMethodField you can simply posts_objects.count in serializer:

class TagSerializer(serializers.ModelSerializer):
    posts_objects = serializers.HiddenField()
    posts = serializers.IntegerField(source="posts_objects.count", read_only=True)


    class Meta:
        model = Tag
        fields = ('name', 'posts_objects', 'posts')

answered Jul 14, 2021 at 8:49

ameen alakhras

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Comments

kamilyrb · Accepted Answer · 2020-02-20 07:22:42Z

1

use this in your serializers.py get_posts

Post.objects.filter(tags__in=[obj]).count()

instead of

obj.post_set.all().count()

answered Feb 20, 2020 at 7:22

kamilyrb

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2 Comments

Abhijit Raj Over a year ago

This does not create a combined category of tags (Sports & Health). I am inclined to think that this task would be better suited on the frontend. I could return a list of posts with their tags from the backend and then count the posts based on the tag combinations in the frontend. Do you agree?

kamilyrb Over a year ago

Yes you can do it, it is not bad way, I think.

Abhijit Raj · Accepted Answer · 2020-02-27 07:01:06Z

0

Currently, I managed to have a working solution by counting the number of objects for every tag in the API result on the frontend. It works for my case, as the data returned is not huge.

let tagDict = {}
let data = response.data
  for (let i = 0; i < data.length; i++) {
    let key = data[i].name
      if ( key in tagDict ) { 
          tagDict[key] += 1
        } else {
          tagDict[key] = 1 
        }   
    }

Marking this as accepted until a better solution comes along.

answered Feb 27, 2020 at 7:01

Abhijit Raj

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Comments

Smokey Akihabara · Accepted Answer · 2022-08-22 15:23:24Z

0

def get_counter(self, obj):
    counter=Post.objects.filter(tags=obj).count()
    return counter

edited Aug 22, 2022 at 15:23

answered Aug 22, 2022 at 15:21

Smokey Akihabara

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1 Comment

Trenton McKinney Over a year ago

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