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]