[Django]-Django accessing ManyToMany fields from post_save signal

If your m2m can be empty (blank=True) you are in a little trouble with m2m_changed, because m2m_changed doesn’t fire if m2m wasn’t set. You can solve this issue by using post_save and m2m_changed at the same time. But there is one big disadvantage with this method – your code will be executed twice if m2m field isn’t empty.

So, you can use transaction’s on_commit (Django 1.9+)

Django provides the on_commit() function to register callback
functions that should be executed after a transaction is successfully
committed.

from django.db import transaction

def on_transaction_commit(func):
    def inner(*args, **kwargs):
        transaction.on_commit(lambda: func(*args, **kwargs))

    return inner

@receiver(post_save, sender=SomeModel)
@on_transaction_commit
def my_ultimate_func(sender, **kwargs):
    # Do things here

Important note: this approach works only if your code calls save().
post_save signal doesn’t fire at all in cases when you call only instance.m2m.add() or instance.m2m.set().

Use transaction on commit!

from django.db import transaction

@receiver(post_save, sender=Project)
def assign_project_perms(sender, instance, **kwargs):
    transaction.on_commit(lambda: print("instance assigned_to: "+str(instance.assigned_to.all())))

here is an example about how to use signal with many to many field (post like and post comments models),

and in my example i have :

• like model (Intermediary table for User and Post tables) : the user can add 1 record only in Intermediary table for each post , which means (unique_together = [‘user_like’, ‘post_like’]) for this type of many to many relations you can use ‘m2m_changed’ signals ,

• comment model (Intermediary table for User and Post tables): the user can add many records in Intermediary table for each post , (without unique_together ), for this i just use ‘post_save, post_delete’ signals , but you can use also ‘pre_save, pre_delete’ if you like ,

and here is both usage example :

from django.db import models
from django.contrib.auth.models import User
from django.db.models.signals import post_save, post_delete, m2m_changed
from django.dispatch import receiver

class Post(models.Model):
    post_user = models.ForeignKey(User,related_name='post_user_related', on_delete=models.CASCADE)
    post_title = models.CharField(max_length=100)
    post_description = models.TextField()
    post_image = models.ImageField(upload_to='post_dir', null=True, blank=True)
    post_created_date = models.DateTimeField(auto_now_add=True)
    post_updated_date = models.DateTimeField(auto_now=True)


    post_comments = models.ManyToManyField(
        User,
        through="Comments",
        related_name="post_comments"
        )

    p_like = models.ManyToManyField(
        User, blank=True,
        through="LikeIntermediary",
        related_name="post_like_rel"
        )

class LikeIntermediary(models.Model):
    user_like = models.ForeignKey(User ,related_name="related_user_like", on_delete=models.CASCADE)
    post_like = models.ForeignKey(Post ,related_name="related_post_like", on_delete=models.CASCADE)
    created =  models.DateTimeField(auto_now_add=True)

    def __str__(self):
        return f"{self.user_like} - {self.post_like} "

    class Meta:
        unique_together = ['user_like', 'post_like']

@receiver(m2m_changed, sender=LikeIntermediary)
def like_updated_channels(sender, instance, **kwargs):
    print('this m2m_changed receiver is called, the instance is post id', instance.id)



class Comments(models.Model):
    cmt_user = models.ForeignKey(User, on_delete=models.CASCADE, related_name="related_comments_user") 
    cmt_post = models.ForeignKey(Post, on_delete=models.CASCADE, related_name="related_comments_post")
    cmt_created_date = models.DateTimeField(auto_now_add=True)
    cmt_comment_body = models.TextField()
    cmt_created =  models.DateTimeField(auto_now_add=True)
    cmt_updated =  models.DateTimeField(auto_now=True)

@receiver(post_save, sender=Comments)
def comments_updated_channels(sender, instance, created, **kwargs):
   print('this post_save receiver is called, the instance post id', instance.cmt_post.id)

@receiver(post_delete, sender=Comments)
def comments_deleted_channels(sender, instance, **kwargs):
   print('this post_save receiver is called, the instance post id', instance.cmt_post.id)

notes :

• the instance with ‘m2m_changed’ it is a post object .
• the instance with ‘post_save and post_delete’ it is a comment object

this is just an example , and change it based on your case/requirements.

i hope this helpful