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From Andrus Adamchik <and...@objectstyle.org>
Subject Re: Batch faulting with Cayenne 3
Date Wed, 14 Nov 2007 15:41:37 GMT
The code below should work. Another way is to use multi-step  
prefetching on a root of to-many relationship.

> 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.

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.

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
>>>
>>
>


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