We will make a change in the change_state method from the previous recipe and display a helpful message when the user is trying to change the state that is not allowed by the is_allowed_transition method. Follow these steps to get started:

1. Add the following import at the beginning of the Python file:

from odoo.exceptions import UserError
from odoo.tools.translate import _

2. Modify the change_state method and raise a UserError exception from the else part:

@api.multi
def change_state(self, new_state):
    for book in self:
        if book.is_allowed_transition(book.state, new_state):
            book.state = new_state
        else:
            msg = _('Moving from %s to %s is not allowed') % (book.state, new_state)
            raise UserError(msg)