Add a count field to a django rest framework serializer

Question

I am serializing the built-in django Group model and would like to add a field to the serializer that counts the number of users in the group. I am currently using the following serializer:

class GroupSerializer(serializers.ModelSerializer):
    class Meta:
        model = Group
        fields = ('id', 'name', 'user_set')

This returns the group ID and name and an array of users (user IDs) in the group:

{
    "id": 3,
    "name": "Test1",
    "user_set": [
      9
    ]
}

What I would like instead as output is something like:

{
    "id": 3,
    "name": "Test1",
    "user_count": 1
}

Any help would be appreciated. Thanks.

Lal · Accepted Answer · 2016-01-08 11:05:25Z

82

A bit late but short answer. Try this

user_count = serializers.IntegerField(
    source='user_set.count', 
    read_only=True
)

answered Jan 8, 2016 at 11:05

Lal

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5 Comments

Vaibhav Jain Over a year ago

Awesome answer . . . . this will also work user_count = serializers.ReadOnlyField( source='user_set.count' )

Sassan Over a year ago

Is it possible to filter user_set too? something like: serializers.IntegerField(source='user_set.count', read_only=True, filters={'user_set.x':'y'})

Lal Over a year ago

not sure. these are the available options for a serializer field. i think the best way will be to use serializers.SerializerMethodField() where you can do any type of custom work including filtering

Prashant Rajput Over a year ago

Nice answer.One question will it fire separate queries for each object to get count of onetomany relation ?

Lal Over a year ago

@PrashantRajput. i think that depends on how you have initialized your queryset or get_object() etc with select_related or prefetch_related. you can take a look here. But looks like at the moment you cannot specify in serializer field to use select_related or prefetch_related for source attr. here is an open issue for this. source attribute doc

emre. · Accepted Answer · 2015-10-26 16:48:37Z

52

This should work

class GroupSerializer(serializers.ModelSerializer):

    user_count = serializers.SerializerMethodField()

    class Meta:
        model = Group
        fields = ('id', 'name','user_count')

    def get_user_count(self, obj):
        return obj.user_set.count()

This adds a user_count field to your serializer whose value is set by get_user_count, which will return the length of the user_set.

You can find more information on SerializerMethodField here: http://www.django-rest-framework.org/api-guide/fields/#serializermethodfield

edited Oct 26, 2015 at 16:48

answered Oct 26, 2015 at 12:09

emre.

1,29611 silver badges13 bronze badges

5 Comments

David Brown Over a year ago

Thanks, works like a charm. Just had to change "obj.user_set" to "obj.user_set.all()"

Mad Wombat Over a year ago

maybe obj.user_set.count() or something

Wtower Over a year ago

This answer is very useful when mongoengine is used and reverse relationships are not available.

Elmux Over a year ago

You can remove self from parameters and add @staticmethod decorator on your definition. :)

Vincent Quirion Over a year ago

@Elmux would it change something or would it just be prettier/ more pythonic?

John Pang · Accepted Answer · 2018-10-25 17:38:16Z

4

Everyone's answer looks great. And I would like to contribute another options here - to use @property if-and-only-if you can modify the target model.

Assume you can modify the Group model.

class Group(models.Model):

    @property
    def user_count(self):
        return self.user_set.count

Then you can simply add 'user_count' to fields in your serializer.

I'm using this approach but I'm thinking to switch to the serializer approach as other's answer here. Thanks everyone.

answered Oct 25, 2018 at 17:38

John Pang

2,51327 silver badges26 bronze badges

4 Comments

Amir Savand Over a year ago

I'd rather not touch the model at all. It's the API's responsibility to provide extra functionality and fields.

John Pang Over a year ago

If the count value is needed by both admin and restful, it would be better to add an extra property to the model - ensure both sides are using same code, as well as dont-repeat-yourself.

reza_khalafi Over a year ago

what is user_set ?

John Pang Over a year ago

user_set is a property inside Group model. Just a SET of users belong to Group.

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