Django Rest Framework - Check/manipulate data (by some logic) before saving to database

Question

I am new to Django Rest Framework. I have a serializer like this:

class ReviewSerializer(serializers.ModelSerializer):
    class Meta:
        model = Review
        fields = (
            'user',
            'course',
            'major',
            'title',
            'text',
            'placements',
            'faculty',
            'infrastructure',
            'students',
            'extra_curricular',
            'passing_year',
        )

models.py

class Review(models.Model):
    user = models.ForeignKey(to=User, related_name='reviews')
    course = models.ForeignKey(to=Course, related_name='reviews')
    major = models.ForeignKey(to=Major, related_name='reviews', blank=True, null=True)
    title = models.TextField(db_index=True)
    text = models.TextField(db_index=True)
    placements = models.FloatField(default=0.0)
    faculty = models.FloatField(default=0.0)
    infrastructure = models.FloatField(default=0.0)
    students = models.FloatField(default=0.0)
    extra_curricular = models.FloatField(default=0.0)
    passing_year = models.PositiveSmallIntegerField(default=2014)
    created_on = models.DateTimeField(auto_now_add=True)
    updated_on = models.DateTimeField(auto_now=True)

And the views.py

class ReviewViewSet(viewsets.ModelViewSet):
    """
    College API
    """
    queryset = Review.objects.all()
    serializer_class = ReviewSerializer
    paginate_by = 5

Now,whenever I post something to the API it directly gets saved to the database if the validation passes successfully. What I need is, I want to check and apply some conditions on the data using some business logic before it saves the data to the database. So that if the condition is true only then the data gets saved, or else it returns a error message to the client.

Answer 1

I assume you are using the new version 3.0. Then you need to put your validation logic into your serializer ReviewSerializer (See http://www.django-rest-framework.org/topics/3.0-announcement/#serializers).

So if you want to create a validation of the fields you should overload the def validate_<fieldname>(self, value). For example if you want Review.placements be >= 0.0, you may write

class ReviewSerializer(serializers.ModelSerializer):
[...]
    def validate_placements(self, value):
         if value >= 0.0:
              return value
         raise serializers.ValidationError('placements should be non-negative.')
[...]

If you need more advanced validation, e.g. to validate two dependent fields, then you need to override the def validate(self, attrs).

For example to have created_on <= updated_on

class ReviewSerializer(serializers.ModelSerializer):
[...]
    def validate(self, attrs):
        if attrs['created_on'] <= attrs['update_on']:
            return attrs
        raise ValidationError('Date of creation should be before update date.')
[...]

If you now add some objects over REST-API, the validation will be called automatically. You don't have to override the methods of ModelViewSet.

In the link above and in www.django-rest-framework.org/api-guide/serializers/#validation are more hints.