Validate inlines before saving model

Question

Let's say I have these two models:

class Distribution(models.Model):
    name = models.CharField(max_length=32)

class Component(models.Model):
    distribution = models.ForeignKey(Distribution)
    percentage = models.IntegerField()

And I'm using a simple TabularInline to show Components inside the Distribution admin form:

class ComponentInline(admin.TabularInline):
    model = Component
    extra = 1

class DistributionAdmin(admin.ModelAdmin):
    inlines = [ComponentInline]

So, my goal is to validate if the percentages of all the Components of the Distribution sum 100 before saving it. Sounds simple, so I did:

# ... Inside the Distribution model
def clean(self):
    # Sum of components must be 100
    total_sum = sum(comp.percentage for comp in self.component_set.all())
    if total_sum != 100:
        raise ValidationError('Sum of components must be 100%')

But this will never work work, because in Django all objects are saved before saving its foreign-key or many2many related objects, this is not a flaw, it has a reason: it cannot save the related objects first, because the object to which they are related doesn't have an id defined yet (id is None until the object is saved for the first time in the DB).

I'm sure I'm not the first guy to run into this issue. So, is there a way to accomplish what I'm trying to do? I was thinking maybe a admin hack using TabularInline or ModelAdmin ... ?

Answer 1

Here's an (untested) idea, if you're happy to move the validation from the model to the inline formset:

Subclass BaseInlineFormSet and override the clean method to check the sum of the percentages.

from django.forms.models import BaseInlineFormSet
from django.core.exceptions import ValidationError

class ComponentInlineFormSet(BaseInlineFormSet):

    def clean(self):
        """Check that sum of components is 100%"""
        if any(self.errors):
            # Don't bother validating the formset unless each form is valid on its own
            return
        total_sum = sum(form.cleaned_data['percentage'] for form in self.forms)
        if total_sum != 100:
            raise ValidationError('Sum of components must be 100%')

Then use your inline formset in the ComponentInline.

class ComponentInline(admin.TabularInline):
    model = Component
    extra = 1
    formset = ComponentInlineFormSet