#34635: ModelChoiceField with a to_attr that can have an EMPTY_VALUE
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Reporter: Willem Van Onsem | Owner: nobody
Type: Uncategorized | Status: new
Component: Uncategorized | Version: 4.2
Severity: Normal | Keywords:
Triage Stage: Unreviewed | Has patch: 0
Needs documentation: 0 | Needs tests: 0
Patch needs improvement: 0 | Easy pickings: 0
UI/UX: 0 |
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Recently a person asked this question on
[StackOverflow](https://stackoverflow.com/questions/76409430/django-
testing-a-simple-model-with-foreign-key-and-updateview/76409867). Imagine
we have the following models:
{{{
from django.db import models
class Location(models.Model):
location_number = models.CharField(max_length=30, unique=True)
name = models.CharField(max_length=128)
class Project(models.Model):
location = models.ForeignKey(Location, on_delete=models.CASCADE,
to_field='location_number', db_column='location_number',
related_name='projects', verbose_name='Standort')
}}}
Then it is possible to construct a `Location` with an empty string for
`location_number`:
{{{
Location.objects.create(location_number='', name='location_name')
}}}
But now if we construct a `ModelForm`, it will not accept this as valid
option, indeed:
{{{
class ProjectForm(forms.ModelForm):
class Meta:
model = Project
fields = ['location']
}}}
This will render a `<select>` with the values of the `to_field` as values,
so:
{{{
<select name="location">
<option value="">location_name</option>
</select>
}}}
now if the form is submitted, it will pass the empty string for
`location`, but the form field will reject it because the field is
required, and it sees this as an empty value.
The problem is thus a bit of a mismatch between what a form does and the
possible values of a model. Unfortunately I don't see an easy solution for
this. Perhaps the checks framework can at least warn if a `ForeignKey`
refers to a `CharField` or another field that can have an empty value.
--
Ticket URL: <https://code.djangoproject.com/ticket/34635>
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