Objectify and relations

Question

I'm trying to implement a simple GAE service. Particularry I have the Student entity and Category entity. To each Student can be associated one or more Categories. How can I create this relationship using Objectify? THanks

Edit: This is my code. Is It valid?

@Entity
public class Studente {
    static long nextID = 17;

    public static Key<Studente> key(long id) {
        return Key.create(Studente.class, id);
    }

    List<Key<Categoria>> categorie; 

    public Studente() {}

    @Id  Long id;
    @Index String nome;
    @Index String cognome;
    @Index String username;
    @Index String password;

    public Studente(String nome, String cognome, String username, String password) {
        this.nome=nome;
        this.cognome=cognome;
        this.username=username;
        this.password=password;
        categorie = new ArrayList<Key<Categoria>>();
    }

    public static long getNextID() {
        return nextID;
    }

    public static void setNextID(long nextID) {
        Studente.nextID = nextID;
    }

    public List<Key<Categoria>> getCategorie() {
        return categorie;
    }

    public void setCategorie(List<Key<Categoria>> categorie) {
        this.categorie = categorie;
    }

    public void addCategoria(Key<Categoria> k ){
        categorie.add(k);
    }
}

Answer 1

I would suggest having a third entity with indexed references to both entities. That way, you could easily query for every student in a category, or for every category of a student.

@Entity
public class Student { /*...*/ }

@Entity
public class Category { /*...*/ }

@Entity
public class StudentCategory {
    @Id
    private Long id;

    @Index
    private Ref<Student> student;

    @Index
    private Ref<Category> category;

    /*...*/
}

We have a similar setup in our GAE applications, and it has served us well.

See documentation of Ref<?>.

Answer 2

Create a muti-valued indexed field in Student that holds all Category IDs (or Keys):

@Entity
public class Category {
    @Id
    public Long id;  // auto-generated Id of the Category

}

@Entity
public class Student {
    @Id
    public Long id;  // auto-generated Id of the Student

    @Index
    public List<Long> categories;  // put Category Ids into this list
}

Indexed fields can be used in query filters, so you will be able to search for students that belong to certain category.