The behavior of m2m_changed signals does not match the documentation.

[Konstantin Yegupov]

Found in v2.2.11, still appears to be valid for the master version.

The documentation at ​https://docs.djangoproject.com/en/3.0/ref/signals/ says:

"pre_add"
Sent before one or more objects are added to the relation.

But, in fact, pre_add is sent even if *zero* objects are added to the relation (because all the objects to be added already exist in the database).
The pk_set argument will be empty, but the signal will still fire.
The same applies to post_add, pre_remove, post_remove "actions".

Either the documentation needs to be corrected to explain that pk_set might be empty, or the code in related_descriptors.py should not fire the signals when missing_target_ids / old_ids is empty.

See also: https://code.djangoproject.com/ticket/29615

[Mariusz Felisiak]

It's already documented that:

pk_set

For the pre_add and post_add actions, this is a set of primary key values that will be, or have been, added to the relation. This may be a subset of the values submitted to be added, since inserts must filter existing values in order to avoid a database IntegrityError.

IMO it's sufficient, if all PKs already exist then it's empty. I don't think that an explicit note that it may be empty changes anything.

Duplicate of #27462.