Hi,This relationship change issue is a very old one in object modeling and made even more interesting when mapping to a relational database, where typically there is only one database column value that represents both sides of the relationship.
Among the standards for persistence (J2EE CMP, JDO 1, JDO 2, and EJB3) the requirements are all over the map, with little to guide you.
CMP defines the behavior as I understand Cayenne currently implements it. That is, the relationship on the other side is silently changed to be consistent.
JDO 1 is silent on the issue.JDO 2 defines the behavior as "undefined until commit or flush", at which point the relationships on both sides are silently changed to be consistent.
EJB3 is silent, and allows relationships to be inconsistent after commit.
I believe it is tricky to code defensively if you want to manage relationships in memory. The issue is the possibility of updating the relationship from either side. The apparently straightforward technique is to implement the Room.setSite method to call oldRoom.remove(this) and newRoom.add(this). And the Site.remove method to call theRoom.setSite(null) and Site.add method to call theRoom.setSite(this). But this causes recursion, unless you use special add, remove, and set methods, that need to be protected from public callers. That is, define package protected methods uncoordinatedAdd, uncoordinatedRemove, and uncoordinatedSet that don't manage the other side, but are called from within the public- visible implementations of add, remove, and set. But clearly this is a lot of work for developers, so it's nice that the persistence implementation does some of the hard work for you.
"Someone" should write a paper. Craig On Jun 19, 2006, at 1:12 AM, Tomi NA wrote:
On 6/19/06, Marcin Skladaniec <[EMAIL PROTECTED]> wrote:Hello Just run into interesting cayenne feature. This code: rooms = site.getRooms(); rooms.remove(aRoom); would alter the relationship so aRoom.getSite() is now null I'm wondering if this is a desired effect ? This behavior might cause bugs. When someone actually puts code to know the fact of relationship being changed (ie. put code into Room setSite() and Site add/removeFromRooms()/setRooms() methods ) he might be disappointed, as those methods would not run, but the relationship will change...I'm wrestling with this issue myself: I've extended the basic templates so that events are fired on setter calls, but this practice has the exact shortcomings you pointed out. Is there a very good reason why cayenne objects don't fire events on a lower level (circumventing this problem) out of the box? Alternatively, if I expand my object code generation templates further so that objectA.removeFrom(objectB) fires a property change event for it's objectA.getBArray() as well as objectB.getToA() - will this completely solve the problem? t.n.a.
Craig Russell Architect, Sun Java Enterprise System http://java.sun.com/products/jdo 408 276-5638 mailto:[EMAIL PROTECTED] P.S. A good JDO? O, Gasp!
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