Automatically filled field in model
You need override the method save of your model. An overrided method must check the condition and show message
You may set the signal receiver on the post_save signal that does the same like (1).
I have some model where there are date field and CharField with choices New or Done, and I want to show some message for this model objects in my API views if  2 conditions are met, date is past and status is NEW, but I really don't know how I should resolve this.
I was thinking that maybe there is option to make some field in model that have choices and set suitable choice if conditions are fulfilled but I didn't find any information if something like this is possible so maybe someone have idea how resolve this?