"Active Constraints" - just an idea

Question

Hi,
I just crossposted this to JDO central (General Discussion) in case you
are interested in following what goes on over there.
Lets say I have a parent child relationship and the child has a
collection of things whose contents have to be a subset of a collection
on the parent. Removing an element from the collection on the parent
requires removal of the same element in the children's collections. To
do this in JDO, as far as I can tell, I have to query to find all the
children with a reference to the element being deleted, and then for
each child I find, I have to remove the element from its (the childs)
collection. What this means is that I have to haul a cart load of data
out of the database to find out what DELETE statements to issue. Which
isn't great.
Concrete but completely made up example:
public class Catalog {
private Collection lineItems;
private Catalog parent;
public void removeLineItem(LineItem li) {
lineItems.remove(li);
for(Iterator i=getAffectedChildren(li);i.hasNext();) {
((Catalog)i.next()).remove(li);
private Iterator getAffectedChildren(LineItem li) {
Query query = pm.newQuery(Catalog.class,"parent == me &&
lineItems.contains(li)");
query.declareParameters("Catalog me, LineItem li);
Collection results = (Collection) query.execute(this,li);
return results.iterator();
Now maybe there is a better way to do this. If there is please shout now
and stop reading the rest of this post ;-)
But, I would love if I could state this kind of business domain
invariant or constraint or whatever you want to call it in JDO meta data
and have it enforced by the JDO runtime. So the JDO runtime could issue
a single SQL DELETE statement, which a WHERE clauses that does all the
work. Surely better than reading in all this data to make a delete as I
(possibly erroneously) am.
Personally I call this notion active constraints or active invariants,
and I see it applying to parent-child relationships like this and cases
where you have to maintain derived attributes.
Cheers,
Mike

Answer 1

THis can be a problem and I sometimes ignore it but there is an occasion where I have to have the track comming in sequence as they are on the CD. for instance Crash Test Dummies - God Shuffeled His Feet, the tracks are sort of blended into each other, and did not sound right when I first uploaded it, so I done it again but this time I put a number in front of the song in the song title tag.
God Shuffled his feet
2 When I go Out With Artists
And so on, It works quite well for me, hope it helps for you.
JJ