where is bidirectional ? grails one-to-one, testing bidirectional (continue) - grails

Question

class Book {
String title
Date releaseDate
String ISBN
static belongsTo = [person:Person] // it makes relationship bi-directional regarding the grails-docs
}
class Person {
Book book; // it will create person.book_id
String name
Integer age
Date lastVisit
static constraints = {
book unique: true // "one-to-one". Without that = "Many-to-one".
}
}
There is a test which test if it is real bidirectional or not. As i understand it.
public void testBidirectional() {
def person = new Person(name:"person_c1", age: 99, lastVisit: new Date())
def book = new Book(
title:"somebook_c1",
ISBN: "somebook_c1",
releaseDate: new Date()
)
person.setBook (book)
assertNotNull(person.save())
def bookId = person.getBook().id
Book thatBook = Book.get(bookId)
assertNotNull(thatBook.person) // NULL !!!
}
So, i save a person with a book, and then i got that book from db by id. Then from that book i try to get back the person which book should refer to (because it should be bidirectional, right?). Eventually i got null instead of an instance of the person.
The questing is: how to make that test working?

Answer 1

i have found the solution how to get it working, but still can not understand why it does not work without 'refresh', see below:
public void testBidirectional() {
def person = new Person(name:"person_c1", age: 99, lastVisit: new Date())
def book = new Book(
title:"somebook_c1",
ISBN: "somebook_c1",
releaseDate: new Date()
)
person.setBook (book)
def p = person.save()
assertNotNull p
person.refresh() //load the object again from the database so all the changes made to object will be reverted
//person = Person.get(p.id) // BUT this also gets the object from db ...?
def bookId = person.getBook().id
assertNotNull bookId
def thatBook = Book.get(bookId)
assertNotNull(thatBook.person)
}
So, here as you can see i use 'refresh' to get it working, but why it does not work without 'refresh' but with the following line after 'refresh' - this one:
person = Person.get(p.id) // BUT this also gets the object from db ...?
If i just want to get object from database by id, then it would be without bidirectional?

Answer 2

Your problem is probably caused by the way that Hibernate works. Grails used Hibernate under the hood.
Even when you call "save", the object person may (and usually) not saved in database. That's because Hibernate is programmed to optimize the query, so it often waits to perform all query at then end of the Hibernate session.
That means if you don't call "refresh", the book-person relation (person.setBook) is still in memory, but not saved in database. Hence you can't get the book.person from book.
To enforce the save, you can use "refresh" like the previous answer, or use flush:true.
I still not try, but it's very likely that you will produce desired results with:
person.save(flush:true)