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From "Craig Russell (JIRA)" <j...@apache.org>
Subject [jira] Commented: (OPENJPA-370) LoadFetchGroup annotation was not recognized during the fetch1
Date Sat, 03 Nov 2007 21:20:50 GMT

    [ https://issues.apache.org/jira/browse/OPENJPA-370?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel#action_12540001
] 

Craig Russell commented on OPENJPA-370:
---------------------------------------

Joe Grassel commented:
> I'm wondering what the intent of the LoadFetchGroup function was when it was designed.
 The manual states:

> "A field can also declare a load fetch group.  When you access a lazy loaded field for
the first time, OpenJPA makes a datastore trip to fetch that field's data.  Sometimes, however,
you know that whenever you access a lazy field A, you're likely to access lazy fields B and
C as well.  Therefore, it would be more efficient to fetch the data for A, B, and C in the
same datastore trip.  By setting A's load fetch group to the name of a fetch group containing
B and C, you can tell OpenJPA to load all of these fields together when A is first accessed."

> I guess I have a question about the function I'd like clarified:

> What does it mean when B and C are co-fetched in the same datastore trip?  Is the data
just loaded into the entitymanager's datacache and held there until a hit is made on it (when
the application finally reads the entity persistable property for the first time, this would
save an additional hit to the database) or is it genuinely considered eagerly fetched (entity
persistable property field is populated when the entity object is constructed by the find/query
operation?)

It's useful to highlight when the load fetch group behavior is activated: when you access
a lazy loaded field for the first time. This is not done during query or find, but only when
the lazy loaded field is accessed (from the application) and it's not already loaded. 

To your other point, if fields B and C are in field A's load fetch group, then accessing field
A makes is available in the detached instance, and fields B and C are also available in the
detached instance as if they were accessed at the same time as field A was accessed.

> This makes a big difference in what an application programmer should expect.  If the
former, then LoadFetchGroup is just a datastore optimization that doesn't really make B and
C eagerly loaded.  It just saves a datastore trip should they ever be loaded.  That means
that if the entity becomes detached, B and C are not available because they were never accessed
when the entity was managed by the persistence context.

Yes, this does affect the fields that are detached. If fields B and C are loaded, then they
are also loaded in the detached instance. But I'd be careful calling this behavior eager loading.
Eager loading is done for fields based on the fetch plan in effect for a find or query that
first loads the instance into memory. The load fetch group isn't considered here. The load
fetch group is only activated when you access a field that wasn't eagerly loaded.

> The latter, and the function behavior I expected, if data is acquired from the datastore
hit, then I'd expect it to be available for reading from the entity object, even if the field
was not access prior to becoming detached, since active fetch groups (or those referenced
by a load fetch group) effectively nullify the LAZY loading setting on an affected persistable
attribute.  Knowing what behavior to expect is especially important, especially in the situation
where entities are acquired with a transaction-scoped persistence context when then find/query
occurs outside of a transaction.  I'd expect A to be loaded because it was referenced in an
active fetch group, and B and C to be loaded (and referenceable in the entity) due to the
load fetch group setting.

No, here's the difference between active fetch groups and load fetch groups. If you want fields
B and C to be loaded when the instance is first accessed via find or query, then you need
to include B and C in one of the active fetch groups when you execute find or query. If you
want fields B and C to be loaded only when some lazy loaded field is accessed, then put B
and C into a fetch group and define that fetch group as the load fetch group of the lazy loaded
fields that you want to trigger the fetch of fields B and C. 

A slightly different slant on this is that if field A is in some fetch group FG1, and use
of field A requires fields B and C, then any fetch group that includes A (e.g. FG1) should
also include B and C. There's no need for a load fetch group here. 

> Also, I noticed that some of the examples closed the entitymanager in order to test loadfetchgroup
behavior -- what about when an entity is just detached from the persistence context, em.close()
is one way to approach it, but that only works in JSE and JEE: Application Managed Persistence
Contexts.  That's not going to work in Container Managed Persistence Contexts, and detachment
is probably going to be frequently seen by Transaction Scoped persistence contexts, and situations
where entities are serialized across the wire to distinct application components (say, to
an application client, a web service, or via RMIIIOP to a remote application server's ejb/web
container.)  I would expect that data to be available due to the fetchgroup/loadfetchgroup
configuration.  

The test cases use em.clear() or em.close() to detach the instances, but any operation that
detaches instances, including serialization, should exhibit the behavior.

> This includes both non-relational and relational lazy-loaded fields.

I don't understand this comment. Are you referring to relationship fields?


> LoadFetchGroup annotation was not recognized during the fetch1
> --------------------------------------------------------------
>
>                 Key: OPENJPA-370
>                 URL: https://issues.apache.org/jira/browse/OPENJPA-370
>             Project: OpenJPA
>          Issue Type: Bug
>          Components: kernel
>    Affects Versions: 1.0.1, 1.0.2, 1.1.0
>            Reporter: Teresa Kan
>            Assignee: Teresa Kan
>             Fix For: 1.0.2, 1.1.0
>
>         Attachments: OPENJPA_370_2.patch, smime.p7s, TestFetchGroup.zip, TestJIRA370.zip
>
>
> Employee class has a LoadFetchGroup annotation defined on the Rating field, when getRating
was called, the address should be returned also. However, openjpa did not handle the LoadFetchGroup
correctly, therefore, address was not eargly fetched.
> public class FGEmployee{
>     @Id
>     private int id;
>  
>     @OneToOne(fetch=FetchType.LAZY) 
>     private FGAddress address;
>  
>     @Basic(fetch=FetchType.LAZY)
>     @LoadFetchGroup("AddressFetchGroup")
>     private String rating;
>  
>     @ManyToOne(fetch=FetchType.LAZY)
>     private FGManager manager;
> ..
> }

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