previous values of ManyToManyField in model's save method

[s4mmael@…]

Hello,

I'm terribly sorry for bothering you, but I believe this behavior may be incorrect. I've already asked at django-users mailing list and I've had no answer.

How to reproduce it:

    class Tag(models.Model):
        name = models.CharField(max_length=50, unique=True, db_index=True)
        parent = ForeignKey('self', null=True, blank=True, related_name='children')

    class Album(models.Model):
        name = models.CharField(max_length=64, blank=True)
        tags = models.ManyToManyField(Tag, blank=True)
        def tags_(self):
            return ', '.join([t.name for t in self.tags.all()])
        def save(self, *args, **kwargs):
            super(Album, self).save(*args, **kwargs)
            f = open('/tmp/test.txt', 'wt')
            f.write('%s\n%s\n' % (self.name, self.tags_()))
            f.close

Now let's go to django admin and create two tags: tag1, tag2. After that let's create an album 'Album_1' with tag 'tag1' and save the album. In the file /tmp/test.txt we see:

---

Album_1

---

Let's add tag2 to the album, change it's name to Album_2 and save. The file will be:

---

Album_2
tag1

---

Let's remove both tags and save:

---

Album_2
tag1, tag2

---

Let's save it again:

---

Album_2

---

So, while everything looks good in Django admin, we get old values of album.tags_() method in Album.save(), even if we call super(Album, self).save(*args, * *kwargs) before.

I've also tried to use signals like this:

    @receiver(post_save, sender=Album)
    def album_save_handler(sender, instance, **kwargs):
        f = open('/tmp/test.txt', 'wt')
        f.write('%s\n%s\n' % (instance.name, instance.tags_()))
        f.close

And even like this:

    @receiver(post_save, sender=Album)
    def album_save_handler(sender, instance, **kwargs):
        t = Album.objects.get(pk = instance.id).tags_()
        f = open('/tmp/test.txt', 'wt')
        f.write('%s\n%s\n' % (instance.name, t))
        f.close

Still no luck, results are the same: previous tags instead of the current ones.

[Russell Keith-Magee]

You've been caught by a leaky abstraction. M2M relationships aren't saved as part of the save() method.

In the admin, the main object is saved, and then the m2m relation is saved; so, by serializing the list of tags in the save method, you're printing the value of the tags before the new values have been saved.

If you want to install "post m2m save" behavior, you'd need to override the update view on the admin itself.

[anonymous]

Russellm, thank you very much for your response! Since I'd read your explanation it was quite simple to find the solution: the 'm2m_changed' signal do the trick.