Hello All,
We've recently upgraded to 2.3.0 (from 1.2.2) and have encountered a nasty bug
where when loading a complicated cyclic object graph using a fetch group, some
relationships are not being populated under some circumstances.
Unfortunately, the graph really is quite complicated, and while I have a set of
test data where the problem can be reliably duplicated, I haven't been able to
make an integration test do the same yet. A simplified version of the graph is
that A is 1:M to B and A is 1:M to C, B is M:1 to A (Bidirectional), and C is
M:1 to A and C is also M:1 to B. The net effect is a graph where cycles can
exist, although in practice they generally don't other than the bidirectional
relationship between A and B.
Our failing case is interesting. When we load single instances of B (call them
B1 and B2) one at a time the load is always successful, with all of the A, B,
and C relationships populated - but only if loaded one at a time. In other
words, if we run entityManager.find(B1) or entityManager.find(B2), then all is
well.
An ascii art version of the graph traversals might look something like this:
B1 -> A1 -> C1 -> B3 -> A3 -> B3 -> A3
-> B3-1 -> A3
-> B3-2 -> A3
-> C1-2 -> ...
....
B2 -> A2 -> C2 -> A3 -> B3 -> A3
-> B3-1 -> A3
-> B3-2 -> A3
-> C2-2 -> ....
...
where B is M:1 to A, A is 1:M to C and A is 1:M to B.
Interestingly, while B1 and B2 are separate objects, they do share several
common objects in their graphs - call them A3 and B3 (as well as B3-1 and
B3-2). If we run a query that loads both B1 and B2 in the same query -
entityManager.find(B1 + B2), then one of the relationships from one of the
other B objects in the graph (call it B3) B3->A is null (not populated), where
B3->A should == A3. To clarify, B1.A1.C1.B3.getA() should equal A3, and
instead is null, and B2.A2.C2.A3.B3.getA() should also equal A3, but instead is
null.
Of course, the graph is being detached after load, so unfortunately lazy
loading the B3->A relationship is not possible.
When running through a debugger, it looks like what happens is that the A3 and
B3-* instances are each being created more than one time. This seems to make
sense because the graph has cycles, relationships are loaded recursively, and
instances are not added to the transactional cache (ManagedCache) until after
they are fully initialized with all fields loaded. Therefore new instances
will not always be fully created when they are needed again. As the call stack
goes forward, the newest A and B instances get fully initialized with all
fields loaded. However, as the call stack unwinds, the oldest A and B
instances are the ones that eventually win, and one of the B instances that
wins is not fully initialized and has a null field in it's A relationship (e.g.
B3.getA() is null, because B3's A was not ever set).
I'm currently tracing this through to try and determine exactly why the outer
(eldest) B3 isn't getting loaded with its A3, but while I do so, I was
wondering if anyone else has encountered a similar problem or has any
suggestions as to where I should focus my efforts.
My current thinking is that the multiple loading issue is OK and expected, and
that the problem is that the oldest B's aren't getting loaded with their A's.
But it is possible that the problem is that each individual entity should only
be initialized once, and that the problem is that OpenJPA is instantiated
multiple copies of each B and A and should only ever be instantiating one per
id. Comments would be welcome.
Thanks,
Jeff
[http://elasticpath.com/images/ep.gif]
Jeff Oh, Sr. Software Engineer
Phone: 604.408.8078 ext. 104
Email: [email protected]<mailto:[email protected]>
Elastic Path Software Inc.
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