Inline Form Validation in Django

The best way to do this is to define a custom formset, with a clean method that validates that at least one invoice order exists.

class InvoiceOrderInlineFormset(forms.models.BaseInlineFormSet):
    def clean(self):
        # get forms that actually have valid data
        count = 0
        for form in self.forms:
            try:
                if form.cleaned_data:
                    count += 1
            except AttributeError:
                # annoyingly, if a subform is invalid Django explicity raises
                # an AttributeError for cleaned_data
                pass
        if count < 1:
            raise forms.ValidationError('You must have at least one order')

class InvoiceOrderInline(admin.StackedInline):
    formset = InvoiceOrderInlineFormset


class InvoiceAdmin(admin.ModelAdmin):
    inlines = [InvoiceOrderInline]

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