So I hate to make this more complicated but ... Yes, this is roughly the right approach. However, if you are creating entities at a rate higher than 1/s, this pattern will break. You'll need to use the sharded counter property:
http://code.google.com/appengine/articles/sharding_counters.html The example is in Python, but hopefully you understand what's happening there. -- Ikai Lan Developer Programs Engineer, Google App Engine plus.ikailan.com | twitter.com/ikai On Fri, Oct 14, 2011 at 9:44 AM, Phil <philippbsee...@googlemail.com> wrote: > Hello, > > I use Java with JDO for the datastore access on google app engine. > > I need the number of rows for the calculation of the number of pages in a > cell table. > This number can be very high so I don't want to calculate it when i query > the data and implemented a class which holds the number of Entities: > > import javax.jdo.annotations.PersistenceCapable; > import javax.jdo.annotations.Persistent; > import javax.jdo.annotations.PrimaryKey; > > @PersistenceCapable > public class Aggregate { > > @PrimaryKey > @Persistent > private String name; > @Persistent > private int count; > public Aggregate(String name) { > this.name = name; > this.count = 0; > } > public String getName() { > return this.name; > } > public void setName(String name) { > this.name = name; > } > public int getCount() { > return this.count; > } > public void setCount(int count) { > this.count = count; > } > } > > To update the count value I do something like that. > > @PersistenceCapable > public class SomeEntity { > @PrimaryKey > @Persistent(valueStrategy = IdGeneratorStrategy.IDENTITY) > private Key key; > ...... > public SomeEntity() { > // increment the count > PersistenceManager pm = PMF.get().getPersistenceManager(); > try { > Aggregate aggregate = (Aggregate) pm.getObjectById(Aggregate.class, > SomeEntity.class.getSimpleName()); > int count = aggregate.getCount(); > count++; > aggregate.setCount(count); > pm.makePersistent(aggregate); > } catch (JDOObjectNotFoundException jonfe) { > // the first "someEntity" ever > Aggregate aggregate = new Aggregate(Player.class.getSimpleName()); > aggregate.setCount(1); > pm.makePersistent(aggregate); > } finally { > pm.close(); > } > } > > ..... > > So far, this works, but I really don't like the way how it's done. Are > there any implemented tools in google app engine to handle something like > that? Or is this the correct way to keep track of the number of entities? > > -- > You received this message because you are subscribed to the Google Groups > "Google App Engine for Java" group. > To view this discussion on the web visit > https://groups.google.com/d/msg/google-appengine-java/-/lK9o1_SMdygJ. > To post to this group, send email to > google-appengine-java@googlegroups.com. > To unsubscribe from this group, send email to > google-appengine-java+unsubscr...@googlegroups.com. > For more options, visit this group at > http://groups.google.com/group/google-appengine-java?hl=en. > -- You received this message because you are subscribed to the Google Groups "Google App Engine for Java" group. To post to this group, send email to google-appengine-java@googlegroups.com. To unsubscribe from this group, send email to google-appengine-java+unsubscr...@googlegroups.com. For more options, visit this group at http://groups.google.com/group/google-appengine-java?hl=en.
