Edit the models/library_book.py file to add the new related field:

1. Ensure that we have a field for the book publisher:

class LibraryBook(models.Model): 
    # ... 
    publisher_id = fields.Many2one( 
        'res.partner', string='Publisher') 

2. Now, add the related field for the publisher's city:

# class LibraryBook(models.Model): 
    # ... 
    publisher_city = fields.Char( 
        'Publisher City', 
        related='publisher_id.city',
        readonly=True) 

Finally, we need to upgrade the add-on module for the new fields to be available in the model.