When should we use the advanced parameters of save()?

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Normally we save an instance into the database simply with inst.save(), but Django uses user.save(using=self._db) in its source code. Also, it uses user.save(update_fields=['last_login']) elsewhere.

This somewhat confuses me. To make things worse, the document for the save() method is extremely simple:

Model.save(force_insert=False, force_update=False, using=DEFAULT_DB_ALIAS, update_fields=None)[source]

If you want customized saving behavior, you can override this save() method. See Overriding predefined model methods for more details.

The model save process also has some subtleties; see the sections below.

It doesn't even contain the explanation of those parameters!

My question is: how do I know when I should use the advanced parameters of save()? If I'm implementing a custom model, I would definitely write user.save().

I've done a couple of experiments myself, like change user.save(using=self._db) to user.save(), and nothing went wrong, but I don't want to be surprised someday. Also, the parameters must be passed for some reasons, right?

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3 Answers

update_fields is also handy when you keep a reference to an instance for a long time (for example in a middleware) and the data might be changed from somewhere else. In these cases you either have to perform a costly refresh_from_db() on the instance to get the newest data from the database, or when you only want to override specific attributes you can omit the refresh_from_db() call and just use save(update_fields=['attr1', 'attr2']). This will leave all other attributes unchanged in the database except for the ones you specified. If you omit update_fields in this case all values would be overwritten in the database with the values of your cached reference to the instance, leading to information loss.

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