AddPrefetch

Sun, 13 Oct 2013 06:06:37 -0700


I came across a rather odd situation and I hope someone can give me some insight into what is going on here. 

 I have a model (diagram attached).  And I'm using this code:

   _expression_ nameExp = ExpressionFactory.matchExp("name",key);
   _expression_ valueExp = ExpressionFactory.matchExp("value",value); 
   SelectQuery pmdQuery = new SelectQuery(PromotionMetaData.class,nameExp);
   pmdQuery.andQualifier(valueExp);
   pmdQuery.addPrefetch("toPromotion")
.setSemantics(PrefetchTreeNode.JOINT_PREFETCH_SEMANTICS);
   pmdQuery.addPrefetch("toPromotion.toPromotionOfferChain");
   @SuppressWarnings("unchecked")
   List<PromotionMetaData> pmdl  = ctx.performQuery(pmdQuery);
   List<UiOfferChain> ofcl = new ArrayList<UiOfferChain>();
   for(PromotionMetaData item: pmdl)
   {
Promotion promo = item.getToPromotion();
if (promo != null)
{
PromotionOfferChain poc = promo.getToPromotionOfferChain();


This example code originally didn't have the calls to addPrefetch and worked fine. However, I know I'm going to iterate through these objects(600), and so I set the prefetch to get them all so that I don't execute over 1200 fetches as I iterate.

 Once the calls where made to AddPrefetch then the code fails with a null pointer exception. 

With the prefetch in place as I singled stepped through the code, I could see Cayenne making the three fetches when the call was made to performQuery.

This line, always returned null. 

Promotion promo = item.getToPromotion();

It wasn't till I added the join semantics that the prefetch worked and the relationships where valid. My question is why?  I've never had to use join semantics to prefetch to work and I wonder what it is about this configuration that requires them? 


Tony 








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