Clarify what's contained in m2m_changed's "pk_set" argument

[Seti]

A code of signal;

@receiver(m2m_changed, sender=Group.user_set.through)
def changed_m2m(sender, instance, action, reverse, model, pk_set, **kwargs):

    print('Signal "{}" | Users in group: {} | pk_set: {}'.format(action, instance.user_set.all(), pk_set))

Run code:

        g1 = Group.objects.all()[0]

        g1.user_set.add(UserModel.objects.all()[0])
        g1.user_set.add(UserModel.objects.all()[0])
        g1.user_set.add(UserModel.objects.all()[0])

        g1.user_set.remove(UserModel.objects.all()[0])
        g1.user_set.remove(UserModel.objects.all()[0])
        g1.user_set.remove(UserModel.objects.all()[0])

Results:

Signal "pre_add" | Users in group: <QuerySet []> | pk_set: {UUID('d0d4a1eb-d2c0-4a27-b9a6-a057a4fa25bc')}
Signal "post_add" | Users in group: <QuerySet [<User: Martin Rivera>]> | pk_set: {UUID('d0d4a1eb-d2c0-4a27-b9a6-a057a4fa25bc')}
Signal "pre_add" | Users in group: <QuerySet [<User: Martin Rivera>]> | pk_set: set()
Signal "post_add" | Users in group: <QuerySet [<User: Martin Rivera>]> | pk_set: set()
Signal "pre_add" | Users in group: <QuerySet [<User: Martin Rivera>]> | pk_set: set()
Signal "post_add" | Users in group: <QuerySet [<User: Martin Rivera>]> | pk_set: set()
Signal "pre_remove" | Users in group: <QuerySet [<User: Martin Rivera>]> | pk_set: {UUID('d0d4a1eb-d2c0-4a27-b9a6-a057a4fa25bc')}
Signal "post_remove" | Users in group: <QuerySet []> | pk_set: {UUID('d0d4a1eb-d2c0-4a27-b9a6-a057a4fa25bc')}
Signal "pre_remove" | Users in group: <QuerySet []> | pk_set: {UUID('d0d4a1eb-d2c0-4a27-b9a6-a057a4fa25bc')}
Signal "post_remove" | Users in group: <QuerySet []> | pk_set: {UUID('d0d4a1eb-d2c0-4a27-b9a6-a057a4fa25bc')}
Signal "pre_remove" | Users in group: <QuerySet []> | pk_set: {UUID('d0d4a1eb-d2c0-4a27-b9a6-a057a4fa25bc')}
Signal "post_remove" | Users in group: <QuerySet []> | pk_set: {UUID('d0d4a1eb-d2c0-4a27-b9a6-a057a4fa25bc')}

If action == 'pre_remove', then pk_set has primary keys of objects which is not presets in a queryset of other model. It is an expected behaviour.

But if action == 'post_remove' then pk_set always contains primary keys of objects, even queryset is empty. Does is expected behaviour?

The Django`s docs says next:
​https://docs.djangoproject.com/en/1.10/ref/signals/#m2m-changed

pk_set
For the pre_add, post_add, pre_remove and post_remove actions, this is a set of primary key values that have been added to or removed from the relation.

So, why the pk_set for the action == 'post_remove' always not empty?

I tested it on the Django 1.10 and the 1.9, the result is same.

[Tim Graham]

I think a documentation clarification indicating that pk_set may be the IDs that are passed in, not necessarily those that are removed, may have to do.

At least, it looks like it would take an extra query to fetch the IDs that are actually removed (and I think accuracy couldn't be guaranteed for pre_remove due to race conditions).

[Carlton Gibson]

​PR

[Carlton Gibson <carlton@…>]

In bed4a152:

Refs #27462 -- Added tests of pk_set in m2m_changed signal receivers for repeated add/remove calls.

[Carlton Gibson <carlton@…>]

In bae05bc:

Fixed #27462 -- Clarifed pk_set difference in m2m_changed signal receivers for add() and remove().

Thank you to Mariusz Felisiak for review.

[Carlton Gibson <carlton.gibson@…>]

In e4dc1b8c:

[3.0.x] Fixed #27462 -- Clarifed pk_set difference in m2m_changed signal receivers for add() and remove().

Thank you to Mariusz Felisiak for review.

Backport of bae05bcf68710cb6dafa51325c3ec83ddda83c39 from master

[Carlton Gibson <carlton.gibson@…>]

In 268e7c77:

[2.2.x] Fixed #27462 -- Clarifed pk_set difference in m2m_changed signal receivers for add() and remove().

Thank you to Mariusz Felisiak for review.

Backport of bae05bcf68710cb6dafa51325c3ec83ddda83c39 from master