The code below should work. Another way is to use multi-step
prefetching on a root of to-many relationship.
Well, that's a good news so we shall probably implement my piece of
code for most relationships. However, I am afraid I didn't understand
your second sentence:-(
Now, it would still be cool if we could have batch faulting for the
odd places where we didn't set up the prefetching.
While I used batch faulting in webobjects days and found it quite
useful, I could never understand how to make it work predictably
(i.e. fault the objects that I care about). I wouldn't object though
to somebody (or even myself) implementing it at the framework level
if whoever that is could explain me the algorithm used to select
which objects to fault. IIRC EOF builds internal "fault chains".
Wonder how much overhead this would incur in Cayenne.
I don't think there is a particular order in which the faults are
fired. Actually, it is not a problem since gradually in a few queries,
all outstanding faults will be fired. My guess is that upon firing the
first fault of a to-one registered to batch fault, you simply (ok, not
that simple:-) look at all entities of same class in the DataContext
and take the first X to be fired at random. X being the size of the
batch. Of course, it some how means if there is a to-one which needs
to be batch faulted it has to be flagged somewhere so you quickly know
which objects to take into account.
BTW relationship prefetching policies can be specified per JPA spec
(and hence will be implemented in Cayenne). However my understanding
is that JPA specifies a different kind of prefetch - which
attributes/relationships to resolve eagerly when an object is fetched.
I am afraid I don't know anything about JPA, but it probably means in
the future some kind of interface in the modeler to be able to specify
those prefetches?
Thanks!
Alex
Andrus
On Nov 14, 2007, at 9:33 AM, Alexander Lamb (dev) wrote:
Well, yes it is possible up to a point.
Usually it is through the "to-many" relationship I get my objects.
Some other times it might be through a custom query meaning I have
to do it each time.
However, as I said in the second email I sent about prefetching,
the solution is maybe the following:
@SuppressWarnings("unchecked")
public List<Role> getRoles () {
if
(org
.apache
.cayenne
.Fault.class.isInstance(this.readPropertyDirectly("roles"))) {
Expression exp = ExpressionFactory.matchExp("person", this);
SelectQuery query = new SelectQuery(Role.class, exp);
query.addPrefetch("profile");
query.addPrefetch("person");
List<Role> roles = getObjectContext().performQuery(query);
ToManyList tml = new ToManyList(this,"roles");
tml.addAll(roles);
writePropertyDirectly("roles",tml);
}
return super.getRoles();
}
The advantage of this is that it does the prefetch but also sets
correctly the "to-many" relationship, meaning it will not refetch
everything if I do an addToRoles or removeFromRoles.
If I want to refault the relationship, I do:
if
(org
.apache
.cayenne
.access
.ToManyList.class.isInstance(this.readPropertyDirectly("roles"))) {
((org.apache.cayenne.access.ToManyList)getRoles()).invalidate();
}
Is this the correct way of doing it?
If so, could there be a way to add this in a generic way to the
model?
Now, it would still be cool if we could have batch faulting for the
odd places where we didn't set up the prefetching.
Alex
Le 14 nov. 07 à 14:45, Andrus Adamchik a écrit :
Can you use prefetching instead? You got a list of users vis some
sort of query - just add prefetch to that query.
Andrus
On Nov 14, 2007, at 8:11 AM, Alexander Lamb (dev) wrote:
Hello list,
One thing is killing performance of our application: it is the
resolving of individual to-one faults in lists.
For example, we can have 200 roles each refering to a person.
When we loop through the roles, for each role where we do a
role.getPerson() there will be a return trip to the database.
In the EOF days, there was a possibility to define a batch
faulting strategy for the entity. In that we would say for
example "batch fault 20 for person" and the first time a to-one
fault to person from role would be found, it would look in the
data context for up to 19 more to build a single SQL statement
and fetch in one go the person objects and resolve up to 20 faults.
Is this feature available somewhere in Cayenne 3m2 or planned in
the near future?
If not, is there some kind of callback or hook wich would allow
us to do the same thing?
Thanks,
Alex
