ManyToMany validation issue.

[Jenna Pullen]

Hi Guys,

I have a m2m relationship (Question - Category) validation issue that I have solved but I am not sure if there is a better way or if I am missing some info from the docs. I have a form that posts a question that has a multi select with existing categories. When I come to save the question in QuestionSerializer my validate_category in CategorySerializer which checks to make sure that the category does not already exist is raising the exception because it does already exist. Which is good when I am actually creating categories in the database, not using an existing one when posting a question. My solution is to just perform the validation in the actual create method instead of the validate_category in CategorySerializer so that I can save a question with existing categories and still check that the category is unique when actually creating a new one?

[Tom Christie]

If would inspect `self.instance` in your validate method. If it exists then an update is being performed and you can exclude `self.instance.pk` from the queryset against which you perform the "does this already exist". This way around any validation errors will be checked during the `is_valid` call, rather than during `.save()`.

[Jenna Pullen]

Thanks Tom, I'll give that a try.

[Jenna Pullen]

Hi Tom,

Unfortunately I think our wires are a little crossed. Your response helps with PUT requests for updates but I still have the validate_category being called when I am creating a new Question and posting existing categories to be added to that question with the Question form. If I move the validation to the save then I can get past the issue. If you picture a form with a question input box and category multi select filled with existing categories. Then when I post the question to save a new question, the new question and existing categories get posted and I then save the question and apply each category to the question. However because I have a validate_category method on the CategorySerializer this seems to be called because I presume it expects that I could be creating new categories at the same time instead of using existing ones posted? Is there a way around this issue? I will post the code so you can get a better picture.

<code>
class CategorySerializer(serializers.HyperlinkedModelSerializer):
    class Meta:
        model = Category
        fields = ('pk', 'category', 'user', 'url')
       
    user = serializers.ReadOnlyField(source='user.username')
   
    def validate_category(self, category):
        if self.instance is not None:
            #update
            category_exists = Category.objects.filter(user=None,
                                                      category__iexact=category)
        else:
            #insert
            category_exists = Category.objects.filter(Q(user=self.context['request'].user) | Q(user=None),
                                                      category__iexact=category)
        if category_exists:
                raise serializers.ValidationError({'category': 'Category %s already exists.'
                                                   % category})
        return category

class QuestionSerializer(serializers.HyperlinkedModelSerializer):
    class Meta:
        model = Question
        fields = ('pk', 'question', 'user', 'url', "categories")

    categories = CategorySerializer(many=True)   
    user = serializers.ReadOnlyField(source='user.username')
   
    def create(self, validated_data):
        categories = validated_data.pop('categories', None)
        question = Question.objects.create(**validated_data)
       
        if categories:
            for category in categories:
                try:
                    # Check if user category already exists and use it.
                    c = Category.objects.get(user=self.context['request'].user,
                                             **category)
                except Category.DoesNotExist:
                    # Check if default category already exists and use it.
                    c = Category.objects.get(user=None,
                                             **category)
                except Category.DoesNotExist:
                    # Category does exist, create new one.
                    c = Category.objects.create(user=self.context['request'].user,
                                                **category)
                question.categories.add(c)
        return question

</code>

[Jenna Pullen]

Anyone come across this before?

On Wednesday, 10 August 2016 14:15:53 UTC+2, Tom Christie wrote:

[Xavier Ordoquy]

Nested serializers don't work with html forms at the moment. Started to work on this is this is still work in progress.

Regards,

Xavier,

Linovia.

[Jenna Pullen]

Thank you for clearing that up for me.