I though its simple what I want, but I cannot find any solution for my problem. I'm using playframework 1.2.3 and it's using Hibernate as JPA. So I think playframework has nothing to do with the problem.

I have some classes (I omit the nonrelevant fields)

public class User {
  ...
}

public class Task {
  public DataContainer dataContainer;
}

public class DataContainer  {
  public Session session;
  public User user;
}

public class Session  {
  ...
}

So I have association from Task to DataContainer and from DataContainer to Sesssion and the DataContainer belongs to a User. The DataContainers can have always the same User, but the Session have to be different for each instance. And the DataContainer of a Task have also to be different in each instance. A DataContainer can have a Sesesion or not (it's optinal). I use only unidirectional assoc. It should be sufficient.

In other words: Every Task must has one DataContainer. Every DataContainer must has one/the same User and can have one Session.

To create a DB schema I use JPA annotations:

@Entity
public class User extends Model {
  ...
}

@Entity
public class Task extends Model {
  @OneToOne(optional = false, cascade = CascadeType.ALL)
  public DataContainer dataContainer;
}

@Entity
public class DataContainer extends Model  {
  @OneToOne(optional = true, cascade = CascadeType.ALL)
  public Session session;

  @ManyToOne(optional = false, cascade = CascadeType.ALL)
  public User user;
}

@Entity
public class Session extends Model {
  ...
}

BTW: Model is a play class and provides the primary id as long type.

When I create some for each entity a object and 'connect them', I mean the associations, it works fine. But when I try to delete a Session, I get a constraint violation exception, because a DataContainer still refers to the Session I want to delete.

I want that the Session (field) of the DataContainer will be set to null respectively the foreign key (session_id) should be unset in the database. This will be okay, because its optional.

I don't know, I think I have multiple problems. Am I using the right annotation @OneToOne ?

I found on the internet some additional annotation and attributes: @JoinColumn and a mappedBy attribute for the inverse relationship. But I don't have it, because its not bidirectional. Or is a bidirectional assoc. essentially?

Another try was to use @OnDelete(action = OnDeleteAction.CASCADE) the the contraint changed from NO ACTIONs when update or delete to:

ADD CONSTRAINT fk4745c17e6a46a56 FOREIGN KEY (session_id)
      REFERENCES annotation_session (id) MATCH SIMPLE
      ON UPDATE NO ACTION ON DELETE CASCADE;

But in this case, when I delete a session, the DataContainer and User is deleted. That's wrong for me.

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EDIT: I'm using postgresql 9, the jdbc stuff is included in play, my only db config is
db=postgres://app:app@localhost:5432/app