I'm wondering what was the rationale for introducing a new separate
signal for m2m relationships, as opposed to using the existing ones on
the intermediate ('through') model. Currently the situations is
* For m2m fields with auto_created intermediate model, calling
``Model.m2m_field.add/remove/clear`` sends m2m_changed. However adding/
removing relations using directly the through model (e.g.
``Model.m2m_field.through.objects.create(...)) does not send
m2m_changed. Adding handlers for pre/post_save, pre/post_delete on the
through model has no effect; the code specifically checks for
auto_created models and does *not* send these signals in this case.
* For m2m fields with a given intermediate model,
``Model.m2m_field.add/remove`` are not exposed (at least for now but
this is under discussion) and therefore m2m_changed is not sent;
instead you have to handle addition/removal in pre/post_save, pre/
post_delete handlers on the through model. However
``Model.m2m_field.clear()`` is exposed and does send m2m_changed; if
there are pre/post_delete handlers on the through model, they are
called as well after the m2m_changed.
If nothing else, this creates an additional discrepancy between
implicit and explicit through models. Switching from one to the other
requires porting all the related signal handlers (a non-trivial task
since m2m_changed has quite different API) and hope for the best. It
also makes things harder for library code that attempts to work on
arbitrary m2m fields, which normally wouldn't (or shouldn't) care
whether the through model is auto_created or not. Have these issues
been raised before ? If so, what's the suggested way to use signals
with m2m relations ?