#33004: Inconsistent / Unexpected handling of assigning unsaved model to Generic
Foreign Key
-------------------------------------+-------------------------------------
Reporter: Finn Andersen | Owner: Jonny
| Park
Type: Bug | Status: assigned
Component: Database layer | Version: dev
(models, ORM) |
Severity: Normal | Resolution:
Keywords: fk, gfk, generic | Triage Stage: Accepted
foreign key, validation |
Has patch: 1 | Needs documentation: 0
Needs tests: 0 | Patch needs improvement: 0
Easy pickings: 0 | UI/UX: 0
-------------------------------------+-------------------------------------
Comment (by wtzhu13):
The code I defined is as follows. I have verified that the data already
exists in the database after I save the object of Grade, but I can not use
it to save the object of Stu.
class Grade(models.Model):
id = models.IntegerField(primary_key=True)
grade = models.CharField(max_length=50, unique=True)
class Stu(models.Model):
id = models.IntegerField(primary_key=True)
name = models.CharField(max_length=50, unique=True)
grade_of_stu = models.ForeignKey(Grade, to_field="grade",
on_delete=models.CASCADE)
###########################################Django
shell##############################################################################
In [17]: g = Grade(grade="3")
In [18]: g.save()
In [19]: stu = Stu(name="Fred", grade_of_stu=g)
In [20]: stu.save()
---------------------------------------------------------------------------
ValueError Traceback (most recent call
last)
<ipython-input-20-a0e3017ded06> in <module>
----> 1 stu.save()
C:\ProgramData\Anaconda3\lib\site-packages\django\db\models\base.py in
save(self, force_insert, force_update, using, update_fields)
680 non-SQL backends), respectively. Normally, they should not
be set.
681 """
--> 682
self._prepare_related_fields_for_save(operation_name='save')
683
684 using = using or router.db_for_write(self.__class__,
instance=self)
C:\ProgramData\Anaconda3\lib\site-packages\django\db\models\base.py in
_prepare_related_fields_for_save(self, operation_name)
930 if not field.remote_field.multiple:
931
field.remote_field.delete_cached_value(obj)
--> 932 raise ValueError(
933 "%s() prohibited to prevent data loss due
to unsaved "
934 "related object '%s'." % (operation_name,
field.name)
ValueError: save() prohibited to prevent data loss due to unsaved related
object 'grade_of_stu'.
In [21]: stu = Stu(name="Fred", grade_of_stu=Grade.objects.get(grade="3"))
In [22]: stu.save()
In [23]:
--
Ticket URL: <https://code.djangoproject.com/ticket/33004#comment:8>
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