Re: Usage of read_only=True with default=serializers.CurrentUserDefault() in 3.8.2

[Carlton Gibson]

Hi Stuart. 

This was a breaking change in v3.8. It was necessary in order to support dotted-sources with nullable fields. (The tl;dr is that having the default set for read-only fields in these cases led to unavoidable inconsistencies, so we had to change it.)

You now need to pass the value of your read-only field when calling save. In your case, something like: 

    def perform_create(self, serializer):

        serializer.save(uploaded_by=self.request.user)

Ultimately the new approach is more consistent. The use of defaults for read-only fields was always a little strange. 

This is mentioned in the 3.8 Release Announcement (and the Release Notes)

http://www.django-rest-framework.org/topics/3.8-announcement/

For **validation** you still need to set the default value as you always have. (This was broken in v3.8.0 but fixed in v3.8.2.) 

Thus this is still correct: 

    default=serializers.CurrentUserDefault()

read-only fields that are part of a unique_together constraint still need some value to validate against, and the default plays that role. 

I hope that explains it. 

Since we’ve made changes here it may be that the docs could do with an adjustment. I think they’re still correct but maybe drawing this out better it needed. It would be good if you could review and offer any suggestions in a PR or Issue on the repo if you think that’s appropriate. 

Kind Regards,

Carlton

On 11 Apr 2018, at 23:30, Stuart George <stu...@accentdesign.co.uk> wrote:

Hi

Ive been using the following with no issues for months, to set the default user and allow it to be sent out in the get requests:

class FileSerializer(serializers.ModelSerializer):
    uploaded_by = serializers.PrimaryKeyRelatedField(
        read_only=True,
        default=serializers.CurrentUserDefault()
    )

    class Meta:
        model = File
        fields = ...

Its used in a basic ModelViewSet and the uploaded_by is not part of any unique validators, it just required.

I just want to check it this is still ok to do this as the uploaded_by is correctly in the serializers data but is missing from the validated_data when its reported as being valid.

Then the save attempt as expected wines with IntegrityError: null value in column "uploaded_by_id" violates not-null constraint.

I can see that there was a unique together validation went in 3.8.2 for read_only + default. Im guessing this is the cause but wanted to check what im doing is still sensible.

Cheers Stu

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[Stuart George]

Hi Carlton,

Yeh thanks I saw the release notes 5 mins after I posted this then deleted the question. 

Thanks for responding, Stu

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Stuart George

Senior Web Developer

Accent Design Group Ltd

The Studio, Adam & Eve Yard

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