[英]ValueError: needs to have a value for field “id” before this many-to-many relationship can be used
[英]ValueError: needs to have a value for field "" before this many-to-many relationship can be used
Django 版本:1.10.4 異常類型: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.
請幫助避免此錯誤:
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)
問題是當我試圖保存 object 時。
保存實例之前,您無法訪問多對多關系的相關對象! 實例必須有一個id
為中間M2N表指向它的外鍵(按名稱schoolbook
,因此錯誤消息):
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)
這是對(令人難忘的)@user2390182 的回答的一個小修正。
我不確定它是什么時候引入的,但是model.save()
方法現在有一個force_insert
參數,它強制執行 SQL INSERT
語句。 但是,這不會在保存前將.pk
設置為 None。
因此,以防萬一設置force_insert=True
(例如, factory-boy
設置),以下代碼有效,而 @user2390182 對 super().save 的第二次調用將引發重復鍵錯誤:
def save(self, force_insert=False, *args, **kwargs):
if not self.pk:
super(SchoolBook, self).save(force_insert, *args, **kwargs)
force_insert = False
# process self.parent_subject (should be called ...subjects, semantically)
super(SchoolBook, self).save(force_insert, *args, **kwargs)
附言。 force_insert
是第一個參數,因此它可以作為位置參數向上傳遞。
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