ManytoMany in playorm

As playorm supports ManytoMany, I have two questions related to this:

How does the entities stored in database in case of ManyToMany. i.e., how many tables/column families are required.
Also, in the examples given in com.alvazan.test.db package, this is being used in Account.java:
```
@NoSqlManyToMany
private CursorToMany<Activity> activitiesCursor = new CursorToManyImpl<Activity>();
```
A similar example is given here . My concern here is that do we always need to use CursorToMany for implementing ManyToMany?

If yes, will it not not cause any issue since CursorToMany does not belong to data layer(Entities will not remain POJO).

If no, then what is the other way round?

Solution

The link you link to shows you the pojo way of many to many but with the pojo method you could get OutOfMemoryExceptions if you have tooooo many in your list which is why we also provide you with the cursor. Let's change the link example you gave me to the following....

public class Student {
  @NoSqlId
  private String id;
  private String firstName;
  private String lastName;
  @ManyToMany
  private List<Course> courses = new ArrayList(); //constructing avoids nullpointers
}

public class Course {
  @NoSqlId
  private String id;
  private String name;
  private String description
  @ManyToOne
  private Lecturer lecturer;
  @ManyToMany
  private List<Student> students = new List<Student>();
}

For any *ToMany(OneToMany, ManyToMany) we store a LIST of foreign keys in the row itself. In the example above, there is ONLY two tables. A row is going to look like this

studentrowkey -> firstName="dean", lastName="hiller", courses:courseFk56=null, courses:courseFk78=null, courses:courseFk98=null, etc. etc.

Notice we did not have it this way

studentrowkey -> firstName="dean", lastName="hiller", courses=courseFk56,courseFk78, courseFk98

The reason we use the former is column operations are independent so if two servers read in the same student...one server may add courseFk200 and another may delete courseFk56 from the student and you both actions will take affect. If you follow the latter method instead one of the server's changes would be lost.

later, Dean