Creating a model with "GeneratedField" results in a DoesNotExist error.

Using Django Mystery Errors
1

class BaseModel(models.Model):

created_at = models.DateTimeField(_("created at"), auto_now_add=True, editable=False)
updated_at = models.DateTimeField(_("updated at"), auto_now=True, editable=False)
created_by = models.ForeignKey(
    settings.AUTH_USER_MODEL,
    verbose_name=_("created by"),
    on_delete=models.PROTECT,
    blank=True,
    null=True,
    editable=False,
    related_name="created_%(class)s",
    db_column="created_by",
)
updated_by = models.ForeignKey(
    settings.AUTH_USER_MODEL,
    verbose_name=_("updated by"),
    on_delete=models.PROTECT,
    blank=True,
    null=True,
    editable=False,
    related_name="updated_%(class)s",
    db_column="updated_by",
)

class Meta:
    abstract = True

class LeadingZeroIDModel(models.Model):

class Meta:
    abstract = True

_id_length = 5 

def save(self, *args, **kwargs):
    with transaction.atomic():
        if not self.id and self._state.adding:
            id_length = kwargs.pop("id_length", self._id_length)
            self.id = self.generate_new_id(id_length)
        super().save(*args, **kwargs)

def generate_new_id(self, id_length):
    last_id = self.__class__.objects.all().values_list("id", flat=True).order_by("-id").first() or "0"
    new_id = str(int(last_id) + 1).zfill(id_length)
    return new_id

class Company(BaseModel, LeadingZeroIDModel):

id = models.CharField(_("id"), primary_key=True, max_length=5, unique=True, editable=False)
name = models.CharField(_("name"), max_length=20, unique=True)
COUNTRY_CHOICES = {
    "JP": _("Japan"),
    "US": _("USA"),
}
country = models.CharField(_("country"), max_length=2, choices=COUNTRY_CHOICES, default="JP")
staff_first_name = models.CharField(_("staff first name"), max_length=35)
staff_last_name = models.CharField(_("staff last name"), max_length=35)

staff_full_name = models.GeneratedField(
    expression=Case(
        When(country="JP", then=Concat(F("staff_last_name"), F("staff_first_name"))),
        When(country="US", then=Concat(F("staff_first_name"), Value(" "), F("staff_last_name"))),
    ),
    output_field=CharField(max_length=70),
    db_persist=True,
)

_id_length = 5

def __str__(self):
    return self.name

class Meta:
    db_table = "company"
    verbose_name = _("company")

When I try to create an object of this model, I get an error
DoesNotExist at company/new/
Company matching query does not exist.

CompanyCreateView is

class CompanyCreateView(CreateView):
model = Company
template_name = “company/company_create.html”
form_class = CompanyCreateForm

def get_success_url(self):
    return reverse_lazy("company-detail", kwargs={"pk": self.object.pk})

def form_valid(self, form):
    form.instance.created_by = form.instance.updated_by = self.request.user
    return super().form_valid(form)

and CompanyCreateForm is

class CompanyCreateForm(forms.ModelForm):

class Meta:
    model = Company
    fields = ["name", "staff_first_name", "staff_last_name"]

If I remove “staff_full_name”, which is a “GeneratedField” from the model, and then migrate it, everything works fine. Why am I getting an error? I am using SQLite in the development phase.

2

You may be hitting one of the SQLite restrictions on generated columns.

I’m thinking #7 is a possibility, but you’d have to check your migrations to see if that’s the issue.

Side note: As a general rule, you typically want to specify mixins before the base classes. In most cases, the intent of a mixin is to supersede the base class. (I’m not certain this is an issue here, but given that you’re overriding the model’s save method in LeadingZeroDModel, I would think that you do want it first in the Company class definition.)

3

Thanks for the comment. KenWhitesell.
Looking at the part

It is not possible to ALTER TABLE ADD COLUMN a STORED column. One can add a VIRTUAL column, however.

I tried changing “db_persist” in GeneratedField to False, but I still get the same error. OK. I guess this doesn’t work with SQLite.

But for something like filtering objects by “staff_full_name”, that field is absolutely necessary (There may be other ways, but none come to mind, at least not for me.).

If I change the database to PostgreSQL or MySQL, will what I want work? Or will I have to try it out to find out?

Addendum: I changed the Company class to class Company(LeadingZeroIDModel, BaseModel) as per your advice. Thanks for your kind reply!

4

What version of Django do you have?
There have been some patch releases for 5.0 around GeneratedFields, just want to check you have the latest patch.

Also minor thing, max_length should be 71 if you do first and last name with a space :+1:

5

Actually, I made a mistake. I missed seeing that both parent classes are abstract. It doesn’t really matter in this case.

I suspect it will. I don’t see anything wrong with the expression that would cause it to fail, provided you’re using the most recent version of Django 5 as suggested above.
(I’d have to try it myself to see.)

I don’t know about MySQL. It won’t in Django unless you’re running 5.1 or apply the patch as described in this thread: Using GeneratedField with Postgres - #2 by felixxm

6

I’m using django version 5.0.2.

That’s right, I was missing this. Thank you :blush:

7

I’ll try it with the most recent version of Django 5 and PostgreSQL, thanks for the advice!