RelatedField(required=False) throws KeyError when the data is missing

[nazarewk]

Checklist

• I have verified that that issue exists against the master branch of Django REST framework.
• I have searched for similar issues in both open and closed tickets and cannot find a duplicate.
• This is not a usage question. (Those should be directed to the discussion group instead.)
• This cannot be dealt with as a third party library. (We prefer new functionality to be in the form of third party libraries where possible.)
• I have reduced the issue to the simplest possible case.
• I have included a failing test as a pull request. (If you are unable to do so we can still accept the issue.)

Actually there is a duplicate but without a valid use case.

User story

I want to fill in the non-nullable relational field on the Model (eg. in save()), i would make it required=False in the DRF to allow it.

Steps to reproduce

from django.db import models
from rest_framework import serializers

class Target(models.Model):
    pass

class Source(models.Model):
    target = models.ForeignKey(Target)

class ExampleSerializer(serializers.ModelSerializer):
    class Meta:
        model = Source
        fields = ('id', 'target')
        extra_kwargs = {'target': dict(required=False)}

serializer = ExampleSerializer(data={})
serializer.is_valid(raise_exception=True)

Expected behavior

assert 'target' not in serializer.data

Actual behavior

try:
    serializer.data
except KeyError:
    print('Nuff said')

[carltongibson]

Can you try this with allow_null please? allow_empty is for lists

[xordoquy]

I'm a bit mixed on that one but would tend to consider that valid.
required is a field core argument so it should be properly handled. However, this rings a bell about doing something wrong here though I can't name it yet.

[nazarewk]

@carltongibson actually looks like neither allow_empty nor allow_null make any difference.

@xordoquy: probably this #3976

[xordoquy]

@nazarewk not really it's a different thing.

[nazarewk]

Well it feels weird for me to set the fields in either Model.clean_fields(), Model.save() or the likes, but sometimes it is extremely hard to avoid.

[carltongibson]

OK. This does seems a problem:

ModelSerializer(data={u'name': u'test'}):
    id = IntegerField(label='ID', read_only=True)
    name = CharField(max_length=100)
    target = PrimaryKeyRelatedField(help_text='Target', queryset=[...], required=False)

We get the KeyError for target here, as per the report. #3984 looks neat enough.

[sdlm]

I have this problem too.

behavior:
https://gist.github.com/sdlm/7890303821aa8cd5c91ea712f8f51be4

#3984 looks good !

[tomchristie]

Okay, thanks for the data point - we'll try to get this reviewed soon.

[tomchristie]

Closing with same resolution as #3984. Happy to reconsider if anyone believes that resolution isn't correct/sufficient.