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?
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