Save to an ArrayList of a model object - arraylist

Question

I am using Play Framework 2.0.4 and in my model class one of the variables is an Array List<Long> datatype, declared here:
public List<Long> associateBooks = new ArrayList<Long>();
I have a view template that invokes a controller method with two Long values that represent the id's of two different objects of the same model type.
GET /addAssociate/:oID/:id controllers.Application.addAssociate(oID: Long, id: Long)
The controller method invoked:
public static Result addAssociate(Long oID, Long id) {
Book.addAssociate(oID, id);
return redirect(routes.Application.index());
}
I know the controller method is performing its action because I get redirected to the index page and I know it receives the values because the URL changes as expected.
The problem lies in the Book model method addAssociate shown below.
public static void addAssociate(Long oID, Long match) {
List<Book> allBooks = new ArrayList<Book>();
allBooks = find.all();
for(Book book: allBooks) {
if(book.id == oID) {
book.associateBooks.add(match);
book.save();
}
}
}
In this method I want to go through all book objects and if the id matches the value of the variable oID passed from the view to the controller I want to add the value of match (the other arguement) to the associateBooks list (of type Long) that belongs to each journey object. I then try and save the changes to the object being considered during this iteration. The problem is the list associateBooks still seems to be empty when I print it out on other view templates.
Update: Even if I remove the condition it still doesn't add anything to the list.

Answer 1

If you want to save your list of associated books, maybe you would like to follow this:
NOTE: I suggest to create additional table named book_associated. It look like this
+--------+-------------------+
|book_id |associated_book_id |
+--------+-------------------+
|1 |2 |
|1 |3 |
+--------+-------------------+
It means that book with ID 1 associated with book with ID 2 and 3
But, this has a disadvantages if Book 1 is associated with Book 2 we cannot determine if Book 2 is also associated with Book 1 because it didn't declared on the table.
So on your model should look like below:
#Entity
#Table(name = "book")
public class Book extends Model {
#Id
public Long id;
// other fields
...
#ManyToMany
#JoinTable(name="book_associated",
joinColumns=
#JoinColumn(name="book_id", referencedColumnName="id"),
inverseJoinColumns=
#JoinColumn(name="associated_book_id", referencedColumnName="id")
)
public List<Book> associateBooks = new ArrayList<>();
// other methods and finder
...
}
Here you can save all your list of associated book of any book. This is for your reference about Many-to-Many relation on java :
http://en.wikibooks.org/wiki/Java_Persistence/ManyToMany
http://www.objectdb.com/api/java/jpa/ManyToMany