Why Sugar orm foreign key is always null

I am using sugar orm in my android project . I am tesing this example :

Author model :

   public class Author extends SugarRecord<Author> {
     String name;
   } 

Book model :

  public class Book extends SugarRecord<Book> {
    String name;
    String ISBN;
    String title;
    String shortSummary;

    // defining a relationship
    Author author;
  }

This is the test case :

Author author = new Author();
author.name = "John";

author.save();

Book book =  new Book();

book.name ="some name";
book.ISBN ="some ISBN";
book.title="some title";
book.shortSummary="short summary";
book.author= author;

book.save();

In this case the author field in the book table is always null.

Where is my mistake ? Please any help