Django m2m_changed pk_set is empty

[Jason Robinson]

Hi everyone,

Django version 1.10.7, I have a problem where sometimes my receiving `m2m_changed` signal on a ManyToMany field has `pk_set` empty.

The model m2m part (note it's a `self` reference):

    class Profile(TimeStampedModel):
        following = models.ManyToManyField(
            "self", verbose_name=_("Following"), related_name="followers"
        )

Signal beginning part with logger:
   

    @receiver(m2m_changed, sender=Profile.following.through)
    def profile_following_change(sender, instance, action, pk_set, **kwargs):
        logger.debug("profile_following_change - sender %s, instance %s, action %s, pk_set %s, kwargs: %s",
                     sender, instance, action, pk_set, kwargs)

Below an example log line with `pk_set` containing the added primary key:

    DEBUG:socialhome:profile_following_change -
        sender <class 'socialhome.users.models.Profile_following'>,
        instance Profile A (profile_a@a.domain.tld),
        action post_add,
        pk_set {2},
        kwargs: {
            'signal': <django.db.models.signals.ModelSignal object at 0x7fa73e033908>,
            'model': <class 'socialhome.users.models.Profile'>,
            'using': 'default',
            'reverse': False
        }

    
And an example with an empty `pk_set`.

    DEBUG:socialhome:profile_following_change -
        sender <class 'socialhome.users.models.Profile_following'>,
        instance Profile B (profile_b@b.domain.tld),
        action post_add,
        pk_set set(),
        kwargs: {
            'model': <class 'socialhome.users.models.Profile'>,
            'reverse': False,
            'using': 'default',
            'signal': <django.db.models.signals.ModelSignal object at 0x7f001c173908>
        }

    
The two saves are both from the same code, using the following line. Note this code runs in an RQ background process, should that matter:

    profile.following.add(user.profile)

Why is the `pk_set` sometimes empty, any ideas? Note that both `Profile` objects also exist before the RQ job is processed (event is triggered by doing a "follow" in the UI).

Also of note, added object *is* found in the ManyToMany field after the operation.

Thankful of any ideas,

Br,
Jason Robinson
https://jasonrobinson.me

[Tim Graham]

You can step through the relevant code here:
https://github.com/django/django/blob/88b290470ab3436ef0bda6b0f3057fe133bda265/django/db/models/fields/related_descriptors.py#L870-L885