Andrus Adamchik created CAY-2877:
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Summary: Don't reset unchanged to-1 relationships of prefetched
objects
Key: CAY-2877
URL: https://issues.apache.org/jira/browse/CAY-2877
Project: Cayenne
Issue Type: Improvement
Reporter: Andrus Adamchik
Consider a chain of to-one relationships: A -> B -> C
{noformat}
// Run a query on B, prefetching C
List<B> b = ObjectSelect.query(B.class).prefetch(B.C.joint()).select(context);
assertFalse(b.get(0).readPropertyDirectly("c") instanceof Fault);
// Run a query on A, prefetching B
A a = ObjectSelect.query(A.class).prefetch(A.B.joint()).selectOne(context);
assertFalse(b.get(0).readPropertyDirectly("c") instanceof Fault); // <-- This
fails
{noformat}
So a somewhat independent query run in the same context can wipe out our
carefully crafted prefetches in the original query, resulting in massive
faulting when you go back and process b's in a loop.
The original idea of blindly invalidating all relationships of a prefetched
object is based on the prefetch contract - all objects in the tree must be
refreshed from the DB. So we are not allowed to keep a stale relationship from
B to C in the second query (e.g. it may have changed from C1 to C2 between the
two queries via an external process). However, we should be smarter, and
preserve at least some relationships, specifically when the fresh B snapshot
has an FK that matches its previous value. So while C itself can become stale,
"B.c" is still pointing to the right C ID. Of course, only works for
relationships that have an FK that has all the target PK columns.
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