Django Signals: Can't Use For Loop? - django

Question

When i remove the for loop in the signals then it works (creates an object properly) but when i use the for loop it should create an object for each post in the Collection object but this doesn't work. It doesn't even create an object of the Collection_List_Item model. Is there a reason why this for loop doesn't work? Is there a way to work around this?
models
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
signals:
#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

For ManyToManyField fields you have to use m2m_changed (Django Docs) signal.
Because ManyToManyField are saved after instance is saved and thus there won't be any record at all of the ManyToManyField updates.
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