Serialize ContentTypes

[Aamu Padi]

I am building an activity model, somewhat similar to this package. It has an actor, verb and the target.

class Activity(models.Model):
    actor_type = models.ForeignKey(ContentType, related_name='actor_type_activities')
    actor_id = models.PositiveIntegerField()
    actor = GenericForeignKey('actor_type', 'actor_id')
    verb = models.CharField(max_length=10)
    target_type = models.ForeignKey(ContentType, related_name='target_type_activities')
    target_id = models.PositiveIntegerField()
    target = GenericForeignKey('target_type', 'target_id')
    pub_date = models.DateTimeField(default=timezone.now)

Now whenever a new object of whichever models (Tender, Job and News) is created, a new Activity object is created, with the target being the objects of any of these three models.

eg. user (actor) published (verb) title (target)

class Job(models.Model):
    user = models.ForeignKey(User)
    title = models.CharField(max_length=150)
    qualification = models.CharField(max_length=100)
    comments = GenericRelation(Comment)
    activity = GenericRelation(Activity)

class News(models.Model):
    user = models.ForeignKey(User)
    title = models.CharField(max_length=150)
    body = models.TextField()
    article_image = models.ImageField(upload_to=get_upload_file_name, blank=True)
    tags = models.ManyToManyField(Tag)
    comments = GenericRelation(Comment)
    activity = GenericRelation(Activity)

class Tender(models.Model):
    user = models.ForeignKey(User)
    title = models.CharField(max_length=150)
    description = models.TextField()
    comments = GenericRelation(Comment)
    activity = GenericRelation(Activity)

To get this data I am making an API which will get me the required json data. Now using django-rest-framework

class ActivitySerializer(serializers.RelatedField):

    def to_representation(self, value):

        if isinstance(value, User):

            return 'Bookmark: ' + value.username

        elif isinstance(value, News):

            return 'Bookmark: ' + value.title

        elif isinstance(value, Job):

            return 'Note: ' + value.title

        elif isinstance(value, Tender):

            return 'Bookmark: ' + value.title

        raise Exception('Unexpected type of tagged object')

But now I am getting this error:

TypeError at /postactivities/

init() takes exactly 1 argument (3 given)

How can I solve this problem? Your help and guidance is very much appreciated.

Thank you.

[Aamu Padi]

Sorry, my bad. I had to use it in this way.

class ActivityObjectRelatedField(serializers.RelatedField):

    def to_representation(self, value):
        if isinstance(value, User):

            return 'User: ' + value.username
        elif isinstance(value, News):
            return 'News: ' + value.title
        elif isinstance(value, Job):
            return 'Job: ' + value.title
        elif isinstance(value, Tender):
            return 'Tender: ' + value.title
        raise Exception('Unexpected type of tagged object')

class ActivitySerializer(serializers.HyperlinkedModelSerializer):
    actor = ActivityObjectRelatedField(read_only=True)
    target = ActivityObjectRelatedField(read_only=True)

    class Meta:
        model = Activity
        fields = ('url', 'actor', 'verb', 'target', 'pub_date')