How to validate objects from reverse side of foreign key - django

Question

I have two objects that are connected together by a ForeignKey.
class Question(models.Model):
text = models.Charfield()
class AnswerOption(models.Model):
text = models.Charfield()
question = models.ForeignKey(Question, on_delete=models.CASCADE, related_name="options")
When Question is created in the admin interface I'm using a Inline form for AnswerOptions so that they can be created at the same time. I'd like to perform some validation on the Question and it needs access to the new AnswerOptions to make the decision.
I've added a clean method to Question but the options property is empty.
What is the correct way to validate Question?
[EDIT]
Made it clear that Question needs access to the AnswerOptions to be able to validate everything.
[EDIT]
Added explicit reference to using an InlineForm for AnswerOptions in the admin interface.

Answer 1

I'd do this through a Django form, which have a more robust interface for
validation. The clean method on your form is the place for this type
of validation.
# forms.py
from django import forms
from .models import Question
class QuestionForm(forms.Form):
text = models.Charfield()
class Meta:
model = Question
def clean(self):
options = self.cleaned_data['options']
if not option.are_ok:
raise forms.ValidationError
# admin.py
from django import admin
from .forms import QuestionForm
class QuestionAdmin(admin.ModelAdmin):
    form = QuestionForm
...
From the docs:
The form subclass’s clean() method can perform validation that requires access to multiple form fields. This is where you might put in checks such as “if field A is supplied, field B must contain a valid email address”. This method can return a completely different dictionary if it wishes, which will be used as the cleaned_data.

Answer 2

This is what I've discovered:
When creating Inline forms in the admin interface Django creates a Formset to handle the multiple forms. (The example here is the same as my use case)
Formsets have a clean() method like other forms and they have a forms property to access the child forms.
Just like normal forms they have an instance property that refers to the 'base' class and the individual forms have an instance property that gets you a instance of the newly submitted data.
Putting it all together:
# models.py
class Question(models.Model):
text = models.Charfield()
class AnswerOption(models.Model):
text = models.Charfield()
question = models.ForeignKey(Question, on_delete=models.CASCADE, related_name="options")
# admin.py
from django.contrib import admin
from django.forms.models import BaseInlineFormSet
class AnswerOptionFormset(BaseInlineFormset):
def clean(self):
super().clean() # See note in docs about calling this to check unique constraints
#self.instance -> Question, with all the newly submitted, and validated, data.
#self.forms -> iterator over all the submitted AnswerOption forms
#for f in self.forms:
# f.instance -> instance of AnswerOption containing the new validated data
#Note: self.instance.options will refer to the previous AnswerOptions
#raise ValidationError for anything that is wrong.
#It is also possible to modify the data in self.instance or form.instance instead.
class AnswerOptionInline(admin.TabularInline):
formset = AnswerOptionFormset # note formset on AnswerOption NOT QuestionAdmin
class QuestionAdmin(admin.ModelAdmin):
inlines = [AnswerOptionInline]