Adding field that isn't in model to serializer in Django REST framework

Question

I have a model Comment that when created may or may not create a new user. For this reason, my API requires a password field when creating a new comment. Here is my Comment model:

class Comment(models.Model):
    commenter = models.ManyToManyField(Commenter)
    email = models.EmailField(max_length=100)
    author = models.CharField(max_length=100)
    url = models.URLField(max_length=200)
    content = models.TextField(blank=True, null=True)
    ip = models.IPAddressField(max_length=45)
    date = models.DateTimeField(default=datetime.now)
    post_title = models.CharField(max_length=200)
    post_url = models.URLField(max_length=200)
    rating = models.IntegerField(max_length=10, default=0)

Here is my API view:

class CommentNewView(CreateAPIView):
    model = Comment
    serializer_class = CommentCreateSerializer

Here is my serializer:

class CommentCreateSerializer(serializers.ModelSerializer):
    commenter_pw = serializers.CharField(max_length=32, required=False)

    class Meta:
        model = Comment
        fields = ('email', 'author', 'url', 'content', 'ip', 'post_title', 'post_url', 'commenter_pw')

Here is the error I am getting:

Environment:


Request Method: POST
Request URL: http://127.0.0.1:8000/api/comment/create/

Django Version: 1.5.2
Python Version: 2.7.2
Installed Applications:
('commentflow.apps.dashboard',
 'commentflow.apps.commenter',
 'commentflow.apps.comment',
 'rest_framework',
 'rest_framework.authtoken',
 'django.contrib.auth',
 'django.contrib.contenttypes',
 'django.contrib.sessions',
 'django.contrib.sites',
 'django.contrib.messages',
 'django.contrib.staticfiles',
 'django.contrib.admin',
 'django.contrib.admindocs')
Installed Middleware:
('django.middleware.common.CommonMiddleware',
 'django.contrib.sessions.middleware.SessionMiddleware',
 'django.middleware.locale.LocaleMiddleware',
 'django.middleware.csrf.CsrfViewMiddleware',
 'django.contrib.auth.middleware.AuthenticationMiddleware',
 'django.contrib.messages.middleware.MessageMiddleware')


Traceback:
File "/Users/tlovett1/.virtualenvs/commentflow/lib/python2.7/site-packages/django/core/handlers/base.py" in get_response
  115.                         response = callback(request, *callback_args, **callback_kwargs)
File "/Users/tlovett1/.virtualenvs/commentflow/lib/python2.7/site-packages/django/views/generic/base.py" in view
  68.             return self.dispatch(request, *args, **kwargs)
File "/Users/tlovett1/.virtualenvs/commentflow/lib/python2.7/site-packages/django/views/decorators/csrf.py" in wrapped_view
  77.         return view_func(*args, **kwargs)
File "/Users/tlovett1/.virtualenvs/commentflow/lib/python2.7/site-packages/rest_framework/views.py" in dispatch
  327.             response = self.handle_exception(exc)
File "/Users/tlovett1/.virtualenvs/commentflow/lib/python2.7/site-packages/rest_framework/views.py" in dispatch
  324.             response = handler(request, *args, **kwargs)
File "/Users/tlovett1/.virtualenvs/commentflow/lib/python2.7/site-packages/rest_framework/generics.py" in post
  372.         return self.create(request, *args, **kwargs)
File "/Users/tlovett1/.virtualenvs/commentflow/lib/python2.7/site-packages/rest_framework/mixins.py" in create
  50.         if serializer.is_valid():
File "/Users/tlovett1/.virtualenvs/commentflow/lib/python2.7/site-packages/rest_framework/serializers.py" in is_valid
  479.         return not self.errors
File "/Users/tlovett1/.virtualenvs/commentflow/lib/python2.7/site-packages/rest_framework/serializers.py" in errors
  471.                 ret = self.from_native(data, files)
File "/Users/tlovett1/.virtualenvs/commentflow/lib/python2.7/site-packages/rest_framework/serializers.py" in from_native
  867.         instance = super(ModelSerializer, self).from_native(data, files)
File "/Users/tlovett1/.virtualenvs/commentflow/lib/python2.7/site-packages/rest_framework/serializers.py" in from_native
  324.             return self.restore_object(attrs, instance=getattr(self, 'object', None))
File "/Users/tlovett1/.virtualenvs/commentflow/lib/python2.7/site-packages/rest_framework/serializers.py" in restore_object
  852.             instance = self.opts.model(**attrs)
File "/Users/tlovett1/.virtualenvs/commentflow/lib/python2.7/site-packages/django/db/models/base.py" in __init__
  415.                 raise TypeError("'%s' is an invalid keyword argument for this function" % list(kwargs)[0])

Exception Type: TypeError at /api/comment/create/
Exception Value: 'commenter_pw' is an invalid keyword argument for this function

Answer 1

If anyone is curious, the solution is to override the restore_object method and add the extra instance variable to the comment object after it has been instantiated:

def restore_object(self, attrs, instance=None):
        if instance is not None:
            instance.email = attrs.get('email', instance.email)
            instance.author = attrs.get('author', instance.author)
            instance.url = attrs.get('url', instance.url)
            instance.content = attrs.get('content', instance.content)
            instance.ip = attrs.get('ip', instance.ip)
            instance.post_title = attrs.get('post_title', instance.post_title)
            instance.post_url = attrs.get('post_url', instance.post_url)
            return instance

        commenter_pw = attrs.get('commenter_pw')
        del attrs['commenter_pw']

        comment = Comment(**attrs)
        comment.commenter_password = commenter_pw

        return comment

Answer 2

Previous answers didn't work on DRF3.0, the restore_object() method is now deprecated.

The solution I have used is awful but I have not found a better one. I have put a dummy getter/setter for this field on the model, this allows to use this field as any other on the model.

Remember to set the field as write_only on serializer definition.

class Comment(models.Model):
    @property
    def commenter_pw():
        return None

    @commenter_pw.setter
    def commenter_pw(self, value):
        pass

class CommentCreateSerializer(serializers.ModelSerializer):
    commenter_pw = serializers.CharField(max_length=32, write_only=True, required=False)

    class Meta:
        model = Comment
        fields = ('email', 'author', 'url', 'content', 'ip', 'post_title', 'post_url', 'commenter_pw')

Answer 3

Thanks for your own answer, it helped me a lot :)

But I think this is a bit more generic, since I still want to call the method on the super serializer class

def restore_object(self, attrs, instance=None):
    '''
    we have to ensure that the temporary_password is attached to the model
    even though it is no field
    '''
    commenter_pw = attrs.pop('comment_pw', None)
    obj = super(
        CommentCreateSerializer, self
    ).restore_object(attrs, instance=instance)
    if commenter_pw:
        obj.commenter_pw = commenter_pw
    return obj