禁止保存（）以防止因相关对象未保存而导致数据丢失

Question

def contact_create(request):
    if request.method == 'POST':
        form = ContactForm(request.POST)
        if form.is_valid():
            form.save()
            return HttpResponseRedirect(reverse(contact_details, args=(form.pk,)))
    else:
        form = ContactForm()

p3=Place(name='Demon Dogs',address='944 W.Fullerton')restaurant.objects.create(Place=p3,serves_hot_dogs=true,serves_pizza=false)
Traceback（最近调用为last）:
...
valueerror：save（）禁止，以防止由于未保存的相关对象'Place'而丢失数据。

这就是我从视图中获取pk的方式：

my_id = ""
if form.is_valid():
    # deal with form first to get id
    model_instance = form.save(commit=False)
    model_instance.pub_date= timezone.now()
    model_instance.user= current_user.id
    model_instance.save()
    my_id = model_instance.pk

if hourformset.is_valid():
    hourformset.save(commit=False)
    for product in hourformset:
        if product.is_valid():
            product.save(commit=False)
            product.company =  my_id
            product.save()
else:
    print(" modelform not saved")
return HttpResponseRedirect('/bizprofile/success')

Answer 1

这是在Django1.8中引入的。以前，您可以将未保存的实例分配给一对一关系，如果失败，它将被悄悄地跳过。从Django1.8开始，在这种情况下，您将得到错误消息。查看Django 1.7->1.8升级的文档。

上面写着：

将未保存的对象分配给ForeignKey、GenericForeignKey和OneToOneField现在会引发ValueError。

for field in self._meta.concrete_fields:
    if field.is_relation:
        # If the related field isn't cached, then an instance hasn't
        # been assigned and there's no need to worry about this check.
        try:
            getattr(self, field.get_cache_name())
        except AttributeError:
            continue
        obj = getattr(self, field.name, None)
        # A pk may have been assigned manually to a model instance not
        # saved to the database (or auto-generated in a case like
        # UUIDField), but we allow the save to proceed and rely on the
        # database to raise an IntegrityError if applicable. If
        # constraints aren't supported by the database, there's the
        # unavoidable risk of data corruption.
        if obj and obj.pk is None:
            raise ValueError(
                "save() prohibited to prevent data loss due to "
                "unsaved related object '%s'." % field.name
            )