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:

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

class Item(models.Model):
    item_name = models.CharField(max_length=200)
    cost = models.IntegerField()
    item_shop = models.ForeignKey(Shopping)

class Buyer(models.Model):
    buyer_name = models.CharField(max_length=200)
    amount = models.IntegerField()
    buyer_shop = models.ForeignKey(Shopping)

In admin.py:

class ItemInline(admin.TabularInline):
    model = Item

class BuyerInline(admin.TabularInline):
    model = Buyer

class ShoppingAdmin(admin.ModelAdmin):
    inlines = (ItemInline, BuyerInline)

So for example it is possible to buy a bottle of rhum at 10$ and one of vodka at 8$. Mike pays 15$ and Tom pays 3$.

The goal is to prevent the user from saving an instance with sums that don’t match: what has been paid must be the same as the sum of the item costs (ie 10+8 = 15+3).

I tried:

• raising ValidationError in the Shopping.clean method. But the inlines aren’t updated yet in clean so the sums are not correct
• raising ValidationError in the ShoppingAdmin.save_related method. But raising ValidationError here gives a very user unfriendly error page instead of redirecting to the change page with a nice error message.

Is there any solution to this problem? Is client-side (javascript/ajax) validation the most simple?

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.