m2m_changed fires before a clear but after add/remove

[rcatherman]

In db/models/fields/related.py I notice that _add_items and _remove_items perform the db actions and then fire the signal to the m2m_changed listeners. _clear_items fires the signal prior to the db interaction. The result is inconsistent. For my use case, I'd like to have my listener assert a relationship has at least one particular item no matter if it is accidentily cleared. It works on removal, but not on clear because I notice the DELETE happens after my re-INSERT.

I've made the change here and it works as expected now.

[Russell Keith-Magee]

This certainly needs to be decided before 1.2. The current behaviour is deliberate, and documented as implemented; I need to think about this (preferably with a clear head) to determine if the current behavior is the most desirable.

[George Vilches]

Why not do both? We have pre/post-save, and pre/post-delete. So, pre/post-m2m_changed would be very much in line with what Django already has.

[Russell Keith-Magee]

(In [12888]) Fixed #13087 -- Modified m2m signals to provide greater flexibility over exactly when notifications are delivered.

This is a BACKWARDS INCOMPATIBLE CHANGE for anyone using the signal names introduced in r12223.

• If you were listening to "add", you should now listen to "post_add".
• If you were listening to "remove", you should now listen to "post_remove".
• If you were listening to "clear", you should now listen to "pre_clear".

You may also want to examine your code to see whether the "pre_add", "pre_remove" or "post_clear" would be better suited to your application.

[Jacob]

Milestone 1.2 deleted