Source: 📖 Django for Professionals ch10 p158

Primary Keys vs IDs

Django's DetailView treats pk and id interchangeably, but there is a subtle difference. By default, Django internally adds an automatically incrementing id field to every model which is used as the SQL Primary Key for that model. However, the field used as the Primary Key can be changed — to an email, for example — so when pk is referenced it would actually point to a field other than id.

It is safer to reference pk when in doubt.