Check existing user when using POST method

[Trieu Tran Quoc]

When we call api "localhost:8000/user/" (POST method) to add new user, if user's existing (user_id is already existing), api return a response normally.

How we can check if a user's existing and raise an existing error?

Database is Cassandra.

view:

class UserList(APIView):

def post(self, request):
    try:
        # add user
        serializer = UserSerializer(data=request.data)
        if serializer.is_valid(True):
            serializer.save()
            return Response(serializer.data, status=status.HTTP_201_CREATED)
        return Response(status=status.HTTP_400_BAD_REQUEST)
    except:
        print("Unexpected error:", sys.exc_info())
        raise
serializer:
class UserSerializer(serializers.Serializer):
    def create(self, validated_data):
        return User.objects.create(**validated_data)

    def update(self, instance, validated_data):
        instance.user_id = validated_data.get('user_id', instance.user_id)
        instance.gold = validated_data.get('gold', instance.gold)
        instance.save()

    user_id = serializers.CharField(required=True)
    gold = serializers.IntegerField(required=False)
model:
class User(Model):
    user_id = columns.Text(max_length=64, primary_key=True, db_field='user_id')
    display_name = columns.Text(max_length=64)
    gold = columns.Integer(default=0)
    passing_level = columns.Integer(default=0)
    live_num = columns.Integer(default=0)
    create_time = columns.DateTime()
    last_update = columns.DateTime()

[Juan Gutierrez]

Hi Trieu,

You may just be able to update your serializer's "create" method to simply do a database lookup against the validated data:

class UserSerializer(serializers.Serializer):
    def create(self, validated_data):

        if User.objects.filter(**validated_data).exists():
            raise Exception('User already exists')
            # Or return 404...or do something else

        return User.objects.create(**validated_data)

However, that operation sounds more like a "validation" - so it would probably be more appropriate to do so by overriding the serializer's validate method:

class UserSerializer(serializers.Serializer):
    def create(self, validated_data):
        return User.objects.create(**validated_data)



    def validate(self, data):
       data = super(UserSerializer, self).validate(data)
       if User.objects.filter(**validated_data).exists():           raise serializers.ValidationError("User already exists")
       return data

Personally, I would restructure your code a bit to leverage more of the framework's built in functionality.

You could turn your serializer into a ModelSerializer and associate your User model with it (see: http://www.django-rest-framework.org/api-guide/serializers/#modelserializer). Then, update your view to use a CreateModelMixin (see: http://www.django-rest-framework.org/api-guide/generic-views/#createmodelmixin). Even after that, to achieve the desired functionality you described originally, I believe the preferred method would still be to override the serializer's validate method.

[Trieu Tran Quoc]

Thanks Juan,

I try using filter but I got an error: 'ModelQuerySet' object has no attribute 'exists' 

This is also a problem I'm facing when interact with Cassandra from DRF, maybe because I'm using django-cassandra-engine model, not django model.

But I understand your mean, thank you :)

Another question about using filter with multiple params, if you know how to do this, please suggest me.

Thank you very much :)

[Juan Gutierrez]

Hi Trieu,

The AttributeError exception about the "exists" method is most likely due to the django-cassandra-engine model implementation, which, unfortunately, I am unfamiliar with. 

The best advice I can give is review the django-cassandra-engine documentation to see what they say about record existence checks and filtering with multiple parameters. Hopefully, someone else with django-cassandra-engine experience will be able to follow up with you regarding your questions.

[Trieu Tran Quoc]

Thanks Juan, but I can't find anything about this :(