Non-required serializer related fields

[andrewyoung1991]

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.)

Steps to reproduce

create a serializer that subclasses serializers.Serializer with a non-required primary key relation

class SearchSerializer(serializers.Serializer):
    q = serializers.CharField()
    user = serializers.PrimaryKeyRelatedField(queryset=User.objects.all(), required=False)

call the serializer in with a request that doesn't have a user parameter

serializer = SearchSerializer(data={"q": "hello world"})
serializer.is_valid(raise_exception=True)
print(serializer.data)  # <- KeyError 'user' is raised here

Expected behavior

the serializer should validate that the field q exists and ignore validating the user field if it is not supplied in the request data. The an example request

{
  "q": "hello world"
}

should be valid, and user should either be None or not included in the serializer.data

Actual behavior

the serializer raises a KeyError on serializer.data if user is not included in the request data.

[xordoquy]

From the documentation:

Relational fields are used to represent model relationships. They can be applied to ForeignKey, ManyToManyField and OneToOneField relationships, as well as to reverse relationships, and custom relationships such as GenericForeignKey.

As such, using PrimaryKeyRelatedField to create a relation between a ModelSerializer and a regular Serializer should not be used.

[nazarewk]

I encountered exactly the same issue, but managed to get it to fail with ModelSerializer.

Here is the whole PR along with the fix #3984

[lovelydinosaur]

The issue looks valid to me as stated.
Nothing wrong with using relational fields on regular serializer classes.
(If anything I'd far rather see bugs reduced to the plain Serializer case, rather than using ModelSerializer, which just masks what set of fields are being used.)

[lovelydinosaur]

Turns out this isn't valid.
required=False does work on a relational field.
What's going wrong here is that you're calling serializer.data, not serializer.validated_data.
required=False ensures that the field does not have to be present in order for the field to validate incoming data.
However does not indicate that a instance to be serializer can omit the value.