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

This seems to be a side-effect of #5375 (documented in #5489) which now requires a default to be set in order for dot-source fields to return None if any part of the relationship is missing.

Here's a simple example:

class Image(models.Model):
    url = models.URLField()
    ...

class Company(models.Model):
    name = models.CharField(max_length=255)
    image = models.ForeignKey('Image', null=True)
    ...

class CompanySerializer(serializers.ModelSerializer):
    image_url = serializer.CharField(read_only=True, default=None, source='image.url')
    
    class Meta:
        model = Company
        fields = ('name, 'image_url')

c  = Company(name='foo')
s = CompanySerializer(c, data={'name': 'bar'}, partial=True)
s.is_valid(raise_exception=True)
s.save()

Expected behavior

I expect s.data will now contain the complete serialized updated instance, including any read-only fields:

>>> s.data 
{'name': 'bar', 'image_url': None}

The relevant line that excludes anything with a default when partial=True: https://github.com/encode/django-rest-framework/blob/master/rest_framework/fields.py#L473

Perhaps this should make an exception for read_only fields?

Actual behavior

Any fields with default set, including read-only, are omitted from the serialized result:

>>> s.data
{'name': 'bar'}