How can I add an extra property to django-rest-framework's ModelSerializer?

Question

I am creating my first django API with django-rest-framework. I have a list of Posts. Each post can have many Likes. Therefore, I have added a ForeignKey to the Like model, which points to its corresponding Post.

When I retrieve the information of a post, I'd like to get the number of likes that this post has.

Here is my url:

path(BASE_URL + "get/slug/<slug:slug>", GetPost.as_view()),

This is my view:

class GetPost(generics.RetrieveAPIView):
    queryset = Post.objects.all()
    serializer_class = PostSerializer
    lookup_field = "slug"

And this is my serializer:

class PostSerializer(serializers.ModelSerializer):
    likes_amount = serializers.SerializerMethodField(read_only=True)

    def get_likes_amount(self, post):
        return post.likes

    class Meta:
        model = Post
        fields = '__all__'

Unfortunately, as soon as I add the extra property likes-amount to the serializer, the API endpoint stops working and I get the following error: Object of type RelatedManager is not JSON serializable.

I'd like to know how I can extend the ModelSerializer with extra fields like the number of likes of my Post, so that I can get. I have read the documentation but haven't found any information on that topic.

Igor Moraru · Accepted Answer · 2021-06-26 11:58:58Z

1

post.likes returns a queryset with related models. if you want the count of likes use count() on that queryset.

def get_likes_amount(self, post):
    return post.likes.count()

answered Jun 26, 2021 at 11:58

Igor Moraru

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