What methods should a model contain
if you using only for the car then you can put that value in the model save method, 
which will get call every time whenever you will update the data.
When I have a model Car where users can upload a single picture to, I can create a property ImageFile in Django to achieve that. Now I would also like to get the metadata from this file and save them to the database.
Now I'm very unsure where I should place this method a how I should design it. It would be obvious, to have a method like _set_exifdata() or _update_exifdata() that is called every time I set an Image to the model.
Or a method get_exifdata(imagefile) that returns a dict of exifdatas. But should this method be part of the Car model? Actually, I won't need it anywhere else, so it does not make sense to put it in a general helper class. But on the other hand, I would prefer to split methods in "retrieving data" and "setting data", so _update_exif() for example would do both of it in once, and maybe that's OK in design ways, but maybe its not and there is some rules of model design I should know and respect here.
I hope someone can help me with some guidelines for model design (especially for django) and what methods should be part of methods and which should not.