ValueError：必须先使用字段“”的值，然后才能使用这种多对多关系

Django版本：1.10.4异常类型：ValueError

Exception Value:    
"" needs to have a value for field "schoolbook" before this many-to-many relationship can be used.

请帮助避免此错误：

class Subject(models.Model):
    name = models.CharField(max_length=256)
    slug = models.SlugField(max_length=256, blank=True)

def __str__(self):
    return self.name

def save(self, *args, **kwargs):
    if not self.id:
        self.slug = slugify(self.name, separator='_')
    return super(Subject, self).save(*args, **kwargs)

class SchoolBook(models.Model):
    parent_subject = models.ManyToManyField(Subject)
    year = models.SmallIntegerField()
    name = models.CharField(max_length=256,blank=True)
    autors = models.CharField(max_length=256)
    slug = models.SlugField(max_length=256, blank=True, unique=True)
    f_file = models.FileField(upload_to=directory_path)

def __str__(self):
    return '{0} {1} {2}'.format(
        self.name, self.autors, self.year)

def save(self, *args, **kwargs):
    if not self.id:
        subject_name = [self.parent_subject.values()[i]['name'] for i in range(len(self.parent_subject.values()))]
        self.name = ' '.join(subject_name)
        self.slug = slugify("{} {} {} {} {}".format(self.name, self.autors,self.year, self.pages, self.c_class))
    return super(SchoolBook, self).save(*args, **kwargs)

问题是当我尝试保存对象时。

保存实例之前，您无法访问多对多关系的相关对象！该实例必须具有一个id中间m2n表的，以将其外键（名称为schoolbook，因此显示错误消息）指向：

def save(self, *args, **kwargs):
    if not self.id:
        super(SchoolBook, self).save(*args, **kwargs)
    # process self.parent_subject (should be called ...subjects, semantically)
    super(SchoolBook, self).save(*args, **kwargs)