小编Art*_*and的帖子

Django版本：1.10.4异常类型：ValueError

Exception Value:    
"<SchoolBook:  0 test1 Test2 Test3 2011 122>" 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} …