Thanks for the responses! I think I'm going to try the "pre-iterate" approach as it will allow me to handle exceptions in the controller tier instead of in the presentation tier.
I think that sort of flexibility would be greatly appreciated by people trying to tune their castor-based applications. Speaking of tuning...it would obviously be faster to execute one SQL statement that brought back the parent and all the children when a relationship field is NOT lazily loaded instead of issuing what appears to be one query for the parent and n queries for the children. Has anyone raised this topic as a potential change already? It seems like a significant performance issue if you have any number of child objects in a relationship at all... --- Werner Guttmann <[EMAIL PROTECTED]> wrote: > > Jon, > > just picking up on one of your questions below, as > Patrick has provided answers to everything else. > > Werner > > On Thu, 6 May 2004 17:22:43 +0100, Patrick van Kann > wrote: > > >Hello Jon, > > > >Unfortunately, lazy loading can only be used in > short transactions. > > > >The underlying RelationCollection holds a > relationship to the transaction context in which the > parent object is loaded, which it calls upon to load > > objects when iterator.next is called. If you have > closed the transaction that loaded the parent > object, it can't lazily load when you call > iterator.next(). > > > >One strategy is to "pre-iterate" in the > transaction, which could work in a situation where > you are providing "pages" of results from a database > query. In > other words, iterate the first 10 items in the lazy > loaded collection before closing the transaction and > returning the object. > > > >Otherwise, you need to load the object from within > your JSP page/presentation tier. If it is jsp, you > could use the castor taglibrary to do this quite > easily: > > > >http://castor-taglib.sourceforge.ne > > > >Hope this helps, > > > >Patrick > > > > > >-----Original Message----- > >From: Jon Wilmoth [mailto:[EMAIL PROTECTED] > >Sent: Thu 5/6/2004 4:59 PM > >To: [EMAIL PROTECTED] > >Subject: [castor-dev] [JDO] lazy loaded collections > > > > > >After turning on logging on my JDBC driver, I > noticed > >that a seperate query statement was issued for each > >child object in a parent child relationship (i.e. > >ProductGroup --> Product). > > > >Query "SELECT > >AA_PRODUCT_GROUP.PROD_GROUP_ID,AA_PRODUCT.PRODUCT_ID,AA_PRODUCT_GROUP.CATEGORY > >FROM AA_PRODUCT_GROUP LEFT OUTER JOIN AA_PRODUCT ON > >AA_PRODUCT_GROUP.PROD_GROUP_ID=AA_PRODUCT.PRODUCT_GROUP_ID" > > execution time: 0 result set fetch time: 0 > > > >Followed by... > > > >Query "SELECT > >AA_PRODUCT.NAME,AA_PRODUCT.PRODUCT_GROUP_ID FROM > >AA_PRODUCT WHERE AA_PRODUCT.PRODUCT_ID=1" > >execution time: 70 result set fetch time: > >0 > >Query "SELECT > >AA_PRODUCT.NAME,AA_PRODUCT.PRODUCT_GROUP_ID FROM > >AA_PRODUCT WHERE AA_PRODUCT.PRODUCT_ID=2" > >execution time: 0 result set fetch time: > >0 > >Query "SELECT > >AA_PRODUCT.NAME,AA_PRODUCT.PRODUCT_GROUP_ID FROM > >AA_PRODUCT WHERE AA_PRODUCT.PRODUCT_ID=3" > >execution time: 70 result set fetch time: > >0 > >Query "SELECT > >AA_PRODUCT.NAME,AA_PRODUCT.PRODUCT_GROUP_ID FROM > >AA_PRODUCT WHERE AA_PRODUCT.PRODUCT_ID=4" > >execution time: 10 result set fetch time: > >0 > > > >In some instances I would want to load the children > >(i.e. a ProductGroup detail page) and other cases I > >wouldn't want to spend time loading the children > (i.e. > >a ProductGroup search results page that didn't > display > >any Product info). So...my first question, is > there a > >way to specify lazy loading of a field at runtime? > No, there isn't. Imho, it would be an interesting > exercise to add such a feature, but I personally > believe that the calling semantics of such a feature > > would be quite hard to specify. > > >In an effort to optimize, I turned the field "lazy" > >attribute value from false to true in the > ProductGroup > >mapping, which resulted in just the first sql query > >being issued...so far so good. However, after > closing > >the query via a database.close() call, and > returning > >the ProductGroup object to the presentation tier, I > >got the following exception when trying to access > the > >ProductGroup's Products: > > > >java.lang.RuntimeException: Transaction is closed! > > > >org.exolab.castor.persist.RelationCollection$IteratorImp.lazyLoad(RelationCollection.java:282) > > > >org.exolab.castor.persist.RelationCollection$IteratorImp.next(RelationCollection.java:266) > > > >Which leads me to my second question: How are lazy > >loaded collections properly used in a web app > >environment? Am I supposed to leave the database > >object open until the presentation tier is done > with > >the query. Am I supposed to loop through the > search > >results (effectively loading them) before returning > >them to the presentation tier? > > > >Thanks, > >Jon > > > > > > > >----------------------------------------------------------- > > >If you wish to unsubscribe from this mailing, send > mail to > >[EMAIL PROTECTED] with a subject of: > > unsubscribe castor-dev > > > > > > > > > > > > ----------------------------------------------------------- > > If you wish to unsubscribe from this mailing, send > mail to > [EMAIL PROTECTED] with a subject of: > unsubscribe castor-dev ----------------------------------------------------------- If you wish to unsubscribe from this mailing, send mail to [EMAIL PROTECTED] with a subject of: unsubscribe castor-dev
