Lets suppose I have a parent child relationship

1 Parent -> n Children

If I want to edit the Parent, I would not load all his children (I just want to set some fields).

Since Parent.getChildren() == null, does Hibernate kills all the relationship when I update the object?

Suppose also that 1 Parent has 1,000,000 children, and I want to add one clhid to that count. Do I have to load all the 1,000,000 children in teh collection, add 1, ad then update?

I mean, if u need to do this, it can become quite expensive dealing with collections.

Reread the documentation on lazy loading.

If you map the collection of children as lazy="true" then you can load the parent, change some properties and resave it without having to load the children (unless you reference the collection of children for some reason).

You can add a child to a bag without the bag being initialized, but I believe you can not add a child to a set without the set being initialized because it doesn't allow duplicates and the return result from Set.add() needs to be calculated.

Well, I could do the opposite also. Building an bidirectional association. So I persist the child passing the Parent as a parameter (with only the id set) instead of loading the parent, adding a child and persisting the parent.

This is the purpose of inverse="true"
read http://www.hibernate.org/117.html#A8 and http://www.hibernate.org/155.html