PATCH with no data is updating ListField values on ModelSerializer instead of leaving them alone

[picturedots]

I've got a model serializer with two ListFields and an email field in it, something like this

class MySettingsSerializer(ModelSerializer):
    location_codes = serializers.ListField(child=serializers.CharField(), 
        required=False)
    permissions = serializers.ListField(child=serializers.CharField(), 
        required=False )

    class Meta:
        model = MySettings
        fields = ('id', 'email', 'location_codes', 'permissions' )

I've also got a view class that uses UpdateModelMixin to provide an update method.

When I do a PATCH to to my view, the expected behavior is that only the values that are sent will be changed. For example, PATCH with
{"permissions": [ "change_mymodel" ] }
should change the permissions field on the MySettings object, and not change any of the other fields. This works as expected.

However, if I do a PATCH with no data at all, both the permissions and locations_codes are set to empty lists. The other field (email) is not changed, which leads me to believe that there is bug related to identifying if a ListField should be updated during a PATCH.

Note that this issue cannot be triggered by the HTML form interface by PATCHING a {} -- I can only see it by using a client like Postman and doing a PATCH with no content.

[tomchristie]

Sounds valid - it'd be good to pull together a more minimal example case that uses a regular serializer rather than a model serializer - that'd help us progress the issue.

[adamsc64]

Hello from DjangoCon sprints. Working on proof of concept solution for this issue. Should have a PR to you in a few days.

[xordoquy]

@adamsc64 Welcome onboard ! It would be wonderfull to start with a failing test case first in order to prove the issue and make sure the fix actually fixes the issue.

[adamsc64]

@xordoquy @tomchristie: I've submitted PR #3415 linked above. Let me know what you think or what else you need from me!