Inconsistent model save behavior when assigning model instances to CharFields

[adsva]

Hi,

When I assign a model instance to a CharField on a model and save the instance, it gets saved as the string representation of the instance on insert, but as the string representation of the instance's pk on update.

from django.db import models

class Country(models.Model):
  name = models.CharField(max_length=100)
  def __unicode__(self):
    return name

class Address(models.Model):
  country = models.CharField(max_length=100)

>>> c = Country.objects.get(name='Sweden')
>>> a = Address.objects.create(country=c)
>>> Address.objects.get(pk=a.pk).country
>>> u'Sweden'
>>> a.save()
>>> Address.objects.get(pk=a.pk).country
>>> '1'

I do realize I should assign c.name instead if c, but it feels like a bug to treat values differently on insert vs update.

The reason seems to be that SQLUpdateCompiler.as_sql preps the value using val.prepare_database_save if available, whereas SQLInsertCompiler.as_sql always uses field.get_db_prep_save.