Django 信号：不能使用 for 循环？

Question

当我删除信号中的for循环时，它可以工作(正确地创建对象(，但当我使用for循环时它应该为Collection对象中的每个post创建一个对象，但这不起作用。它甚至不创建Collection_List_Item模型的对象。这个for循环不起作用是有原因的吗？有办法解决这个问题吗？

型号

class Collection(models.Model):
posts = models.ManyToManyField(Post, related_name='collection_posts', blank=True)
author = models.ForeignKey(User, on_delete=models.CASCADE, null=True)
collection_name = models.CharField(max_length=100)
collection_description = models.CharField(max_length=1000, blank=True)
collection_likes = models.ManyToManyField(User, related_name='liked_collections', blank=True)
collection_image = models.ImageField(upload_to="images/")
private = models.BooleanField(default=False)
follows = models.ManyToManyField(User, related_name='collection_follows', blank=True)
def __str__(self):
return self.collection_name
class Collection_List_Item(models.Model):
post = models.ForeignKey(Post, on_delete=models.CASCADE, null=True)
user = models.ForeignKey(User, on_delete=models.CASCADE, null=True)
collection = models.ForeignKey(Collection, on_delete=models.CASCADE, null=True)
saved = models.BooleanField(default=False)
created_date = models.DateTimeField(auto_now_add=True)
modified_date = models.DateTimeField(auto_now=True)
def __str__(self):
return self.collection.collection_name

信号：

@receiver(post_save, sender=Collection)
def create_collection_list_item(sender, instance, created, **kwargs):
if created:
for i in instance.posts.all():
collection_list_item = Collection_List_Item.objects.create(collection=instance, user=instance.author, post=i)

collection_list_item.save()

Answer 1

对于ManyToManyField字段，必须使用m2m_changed(Django Docs(信号。

因为ManyToManyField是在实例保存后保存的，因此在所有ManyToManyField更新时都不会有任何记录。

from django.db.models.signals import m2m_changed
from django.db import IntegrityError
@receiver(m2m_changed, sender=Collection.posts.through)
def create_collection_list_item(sender, instance, action, *args, **kwargs):
if action == "post_add":
for i in instance.posts.all():
try:
collection_list_item = Collection_List_Item.objects.create(collection=instance, user=instance.author, post=i)
except IntegrityError:
pass