I feel like I am having a kind of brain cramp. I have an entity that has a parent/child relationship to another. In this case its Episode and Comments. I am using @OneToMany and @ManyToOne respectively. The @OneToMany has mappedBy="episode" set.

I have a web framework (struts2) that will create an Entity by calling the default constructor and call the appropriate setters based on posted parameters.

In the past I swear that if I did not load the object from the db, then the Comments collection gets initialized to an empty collection in the constructor and that empty collection overwrites the existing Comments.

For some reason I am not seeing this behavior and after a lot of testing, googling, etc it looks like if I create an object, set the ID to an existing persistant object and then call getHibernateTemplate().merge() -- I am using Spring HibernateDaoSupport -- hibernate sees the empty collection and ignores it leaving the existing comments in tact.

So really this is actually kind of good behavior, unless you wanted to wipe out a collection by setting it an empty instance. I guess its just not what I expected and I am wondering if Hibernate is really smart enough to ignore an empty collection object when the persisted object has items?

I suppose that it would be pretty dangerous if Hibernate allowed you to wipe out a collection by accidently trying to persist an updated object without its collection(s) intialized.....

Can someone verify my theory or educate me?

So I have done some more testing and I think the issue that when I do this:

@OneToMany(mappedBy = "episode")
Set<Comment> comments;

@ManyToOne
Episode episode;

I am telling Hibernate that the Comments side owns this relationship. I could make this unidirectional and I get the behavior I was expecting:

@OneToMany(cascade = CascadeType.ALL)
@JoinColumn(name = "episode_id")
@org.hibernate.annotations.Cascade(
value = org.hibernate.annotations.CascadeType.DELETE_ORPHAN
)
Set<Comment> comments;

but my real question is: How can I make the Parent (Set) side of the relationship the owning side, when its bidirectional?

Hi,
I copied this from http://www.hibernate.org/hib_docs/annotations/reference/en/html_single/:


look at paragraph "2.2.5.3.2.1. Bidirectional" for the complete example.