[Django]-DRF, add custom field to ModelSerializer

try to use SerializerMethodField field as

class TaskSerializer(serializers.ModelSerializer):
    is_in_usertask = serializers.SerializerMethodField(read_only=True)

    class Meta:
        model = Task
        fields = ('name', 'is_in_usertask')

    def get_is_in_usertask(self, task):
        return UserTask.objects.filter(user=self.context['request'].user, goal=task.goal).exists()