最好的方法是定义一个自定义表单集，使用一种干净的方法来验证至少存在一个发票订单。

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]