ValueError with multi-table inheritance in Django Admin
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27-09-2019 - |
Question
I created two new classes which inherit model Entry:
class Entry(models.Model):
LANGUAGE_CHOICES = settings.LANGUAGES
language = models.CharField(max_length=2, verbose_name=_('Comment language'), choices=LANGUAGE_CHOICES)
user = models.ForeignKey(User)
country = models.ForeignKey(Country, null=True, blank=True)
created = models.DateTimeField(auto_now=True)
class Comment(Entry):
comment = models.CharField(max_length=2000, blank=True, verbose_name=_('Comment in English'))
class Discount(Entry):
discount = models.CharField(max_length=2000, blank=True, verbose_name=_('Comment in English'))
coupon = models.CharField(max_length=2000, blank=True, verbose_name=_('Coupon code if needed'))
After adding these new models to admin via admin.site.register I'm getting ValueError when trying to create a comment or a discount via admin. Adding entries works fine.
Error msg:
ValueError at /admin/reviews/discount/add/
Cannot assign "''": "Discount.discount" must be a "Discount" instance.
Request Method: GET
Request URL: http://127.0.0.1:8000/admin/reviews/discount/add/
Exception Type: ValueError
Exception Value:
Cannot assign "''": "Discount.discount" must be a "Discount" instance.
Exception Location: /Library/Python/2.6/site-packages/django/db/models/fields/related.py in set, line 211
Python Executable: /usr/bin/python
Python Version: 2.6.1
Solution
The reason for this error appeared was because I used same column name that was already used with model name. Karen T. contributed the following answer in Django mailing list:
The problem seems to be that you have named a field in your Comment model with the same name, only lower case. Comment inherits from Entry, using multi-table-inheritance. This adds a OneToOneField in Comment back to Entry, which has a side-effect of adding a 'comment' attribute to Entry. This is the attribute that lets you access the Comment associated with the Entry as a result of the OneToOneField in Comment, and by default it is given the name of the related model, all-lowercase.
The problem then occurs when the Comment model "inherits' all the fields/attributes of Entry: the inherited 'comment' attribute from Entry is apparently over-riding the specified comment field. That's probably a bug, but it appears to have been there since 1.0. I have not done any research to see if it's been reported.
As a workaround you can name your fields something other than the model name all lowercased, or you can explicitly specify the OneToOneField in the child models, specifying parent_link=True and something other than the model name all lowercased for related_name.
OTHER TIPS
Hunch says you could do with declaring your Entry class as an abstract one unless you need an Entry as an actual object, too
...rest of Entry model here...
created = models.DateTimeField(auto_now_add=True) ## ONLY set date when created, not every save
class Meta:
abstract = True
...methods for your model etc...