Validation of dependant inlines in django admin

Question

I am using Django 1.4 and I want to set validation rules that compare values of different inlines.

I have three simple classes

In models.py:

Answer 1

You could override your Inline formset to achieve what you want. In the clean method of the formset you have access to your Shopping instance through the 'instance' member. Therefore you could use the Shopping model to store the calculated total temporarily and make your formsets communicate. In models.py:

class Shopping(models.Model):
   shop_name = models.CharField(max_length=200)

   def __init__(self, *args, **kwargs)
       super(Shopping, self).__init__(*args, **kwargs)
       self.__total__ = None

in admin.py:

from django.forms.models import BaseInlineFormSet
class ItemInlineFormSet(BaseInlineFormSet):
   def clean(self):
      super(ItemInlineFormSet, self).clean()
      total = 0
      for form in self.forms:
         if not form.is_valid():
            return #other errors exist, so don't bother
         if form.cleaned_data and not form.cleaned_data.get('DELETE'):
            total += form.cleaned_data['cost']
      self.instance.__total__ = total


class BuyerInlineFormSet(BaseInlineFormSet):
   def clean(self):
      super(BuyerInlineFormSet, self).clean()
      total = 0
      for form in self.forms:
         if not form.is_valid():
            return #other errors exist, so don't bother
         if form.cleaned_data and not form.cleaned_data.get('DELETE'):
            total += form.cleaned_data['cost']

      #compare only if Item inline forms were clean as well
      if self.instance.__total__ is not None and self.instance.__total__ != total:
         raise ValidationError('Oops!')

class ItemInline(admin.TabularInline):
   model = Item
   formset = ItemInlineFormSet

class BuyerInline(admin.TabularInline):
   model = Buyer
   formset = BuyerInlineFormSet

This is the only clean way you can do it (to the best of my knowledge) and everything is placed where it should be.

EDIT: Added the *if form.cleaned_data* check since forms contain empty inlines as well. Please let me know how this works for you!

EDIT2: Added the check for forms about to be deleted, as correctly pointed out in the comments. These forms should not participate in the calculations.

Answer 2

Alright I have a solution. It involves editing django admin's code.

In django/contrib/admin/options.py, in the add_view (line 924) and change_view (line 1012) methods, spot this part:

        [...]
        if all_valid(formsets) and form_validated:
            self.save_model(request, new_object, form, True)
        [...]

and replace it with

        if not hasattr(self, 'clean_formsets') or self.clean_formsets(form, formsets):
            if all_valid(formsets) and form_validated:
                self.save_model(request, new_object, form, True)

Now in your ModelAdmin, you can do something like this

class ShoppingAdmin(admin.ModelAdmin):
    inlines = (ItemInline, BuyerInline)
    def clean_formsets(self, form, formsets):
        items_total = 0
        buyers_total = 0
        for formset in formsets:
            if formset.is_valid():
                if issubclass(formset.model, Item):
                    items_total += formset.cleaned_data[0]['cost']
                if issubclass(formset.model, Buyer):
                    buyers_total += formset.cleaned_data[0]['amount']

        if items_total != buyers_total:
            # This is the most ugly part :(
            if not form._errors.has_key(forms.forms.NON_FIELD_ERRORS):
                form._errors[forms.forms.NON_FIELD_ERRORS] = []
            form._errors[forms.forms.NON_FIELD_ERRORS].append('The totals don\'t match!')
            return False
        return True

This is more a hack than a proper solution though. Any improvement suggestions? Does anyone think this should be a feature request on django?