How to do it...

For defining a method on Library Book to allow changing the state of a selection of books, you need to add the following code to the model definition:

1. Add a helper method to check whether a state transition is allowed:

    @api.model 
    def is_allowed_transition(self, old_state, new_state): 
        allowed = [('draft', 'available'), 
                   ('available', 'borrowed'), 
                   ('borrowed', 'available'), 
                   ('available', 'lost'), 
                   ('borrowed', 'lost'), 
                   ('lost', 'available')] 
        return (old_state, new_state) in allowed 

2. Add a method to change the state of some books to a new one passed as an argument:

    @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: 
                continue