Turns out it is changed in  str method for the User model
For establishing many-to-many relationships, django admin has a nice widget called 'filter_horizontal'.
The problem is - it only displays primary key of the related table.
How do I add other fields from the related table?
For example, if I have a many-to-many relationship in my Order model with User model, in 'Order' django admin I can only see User's primary key(id). How do I add their name into the widget?
Django admin many-to-many - how to show additional fields?