How can I access the 'Action' field in django-simple-history?

Question

I'm using django-simple-history to record changes in my models. I cannot find a way to access the "Action" in the history model.

DATE/TIME USER ACTION

invoice.history_date 
invoice.history_user
invoice. ____ ? 

I have not found a working solution. I have tried: invoice.action, invoice.actions, invoice.history_action/s, etc. and they don't work. I want to show what exactly was changed.