Why is m2m_changed called when there is no connection yet?

Question

Django

There are two related models

class Tag(models.Model):
    pass

class Photo(models.Model):
    tags = models.ManyToManyField(Tag, related_name='tags', blank=True)

This is how I end up:

@receiver(m2m_changed, sender=Photo.tags.through)
def photo_tags_changed(sender, instance, action, **kwargs):
    print( action, kwargs, instance.tags.all())

print output

post_add {'pk_set': {28538}, 'using': 'default', 'reverse': False, 'model': <class 'apps.photos.models.Tag'>, 'signal': <django.db.models.signals.ModelSignal object at 0x7fc890ddda20>} []

As far as I understand, the "post_add" action should take place after the link has been added.
However, a new tag is passed to pk_set, but if you do instance.tags.all(), then the link has not yet been created. I thought maybe something was cached in the ORM, but at that moment I checked in the database, there is no connection yet.
I am using DRF, the serializer looks like this:

class PhotoWriteSerializer(serializers.ModelSerializer):
    class Meta:
        model = Photo

It seems that DRF is somehow affected, tk. if you add tags from the console without it, then there are already links in post_add.

Answer 1

In general, this bug is related to this.
Why does Djnago Rest Framework return old information about related objects on update?