So, if there is one thing that drives me nuts about Cayenne, it's managing
ObjectContexts. In particular, you cannot relate two Persistent objects
without first putting them into an ObjectContext. If one is committed and
the other is not, you can have them in different contexts, but for newly
created objects, this is a major pain in the neck.
Since I've been complaining about it for probably close to three years now,
I'd like to finally do something about it.
Here are my rough notes from the airport:
OK Cases:
- Objects in same context
- Objects in different contexts, but objects are committed already
Don't work, but should:
- Objects in null contexts
- Objects in different contexts, but same data maps and domains
Very hard to say, probably okay if don't work:
- Objects in different contexts, contexts have different data maps
- Objects in different contexts, contexts have different data domains
As I started actually digging into the code, I ran into a lot of NPE issues
trying to associate two Persistent objects with no context with one another.
In an attempt to prevent adding special null-logic handling, I thought about
applying the Null Object pattern to the problem. The basic idea is rather
than use null as the default objectContext for CDO, use an instance of
DelayedContext. The problem here is having objects in different contexts.
So, it appears by fixing one, you can essentially fix the other.
To address the latter, I was looking to have a Set of ObjectContexts stored
in either BaseContext or DataContext. When willConnect() is called, you'd
have something like the following:
else if (this.getObjectContext().getEntityResolver() ==
object.getObjectContext().getEntityResolver()) {
((DataContext)
this.getObjectContext()).addContextToMergeWith(object.getObjectContext());
((DataContext)
object.getObjectContext()).addContextToMergeWith(this.getObjectContext());
}
else {
throw new CayenneRuntimeException(
"Cannot set object as destination of relationship "
+ relationshipName
+ " because it is in a different DataMap or
DataDomain.");
}
(Casts are just an artifact of me screwing around).
What I'm thinking would happen is that when commitChanges() is called, the
set of contexts to be merged with will be iterated and any changes applied
to the current object store / object store graph diff. The relationship is
bidirectional so that the user can initiate the commit from any object
registered with any context.
Here is about where I lose it. I'm not as well-versed in the internal
going-ons of Cayenne as I would like to be. It appears Cayenne goes to
great efforts to essentially cache the graph manipulations so as to avoid a
full traversal. I really don't know, though.
Caching ordering of operations could make this tricky, but in principal
should be wholly doable.
If anyone has any thoughts on this or can fill in any missing pieces, I'd
appreciate it. This is really something I'd like to see fixed sooner rather
than later. I think it may be a requisite for JPA compliance as well.
-- Kevin
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