This situation comes up, from time to time, when we need to get something to happen after a many-to-many field is changed. The novice will connect a post_save signal and scratch his head when it doesn't fire on the addition or removal of items in the ManyToManyField. We all learn that it takes a slightly more complicated signal, the m2m_changed signal, and its many actions , which tell us exactly what has changed in the particular field sending it (the signal comes from the field's through table, to be exact). Well, a slightly more complicated case arose in a design today and I was scratching my head and feeling like a novice all over again. You see, I needed to know when new things had been adding to such a field, but I had more than one. In fact, I had four of them. I needed a specific function called on the instance when all of these fields were finished being cleared or added to or subtracted from. This was in a form in the Django admin. Thankfully I had an assumpti...