How to know which field is saved when only() has been used in Django?
When you use defer or only to load an instance, the get_deferred_fields() method returns a list of field names that have not been loaded; you should be able to use this to work out which ones will be saved.
I am using Django 1.11, in one of my models I have added actions when the model is saved.
However I don't want these actions to be done when only a part of the model is saved. 
I know that update_fields=('some_field',) can be used to specify which field must be saved.
But, when the object has been fetched in the database using the methods only() or defer() I don't see any information about the fields updated in the save() method, update_fields is empty.
Thus my question: How can I get the fields saved by Django when only some fields have been fetched ?