Notice that we have the state field on the library.book model. This field is used to represent the status of the book, or, in other words, whether it is available or not. We have added a book_state field to map the state of the book with the dynamic stages.

In order to reflect the book state from the stage, we need to override the create and write methods in the library.book.rent model, as follows:

    @api.model
    def create(self, vals):
        rent = super(LibraryBookRent, self).create(vals)
        if rent.stage_id.book_state:
            rent.book_id.state = rent.stage_id.book_state
        return rent

    @api.multi
    def write(self, vals):
        rent = super(LibraryBookRent, self).write(vals)
        if self.stage_id.book_state:
            self.book_id.state = self.stage_id.book_state
        return rent

After this, whenever the user changes the stage of any loan record, it will be reflected in the book record.