Hi Kevin,
Relating (as in "creating a direct reference in a Java sense") two
objects belonging to two distinct contexts breaks fundamental Cayenne
design, most notably assumptions about uniquing and transaction
boundaries. I am -1 on that until somebody persuades me that this is
a good idea and explains how to avoid messing up existing
assumptions. (FWIW there is a workaround - referencing a peer of a
given object in another context via 'localObject').
On the other hand the inability to relate two TRANSIENT objects (i.e.
objects without a context) is indeed a shortcuming. Here is one way
how it might work (following the JPA patterns) :
When such relationship is established, we would not attempt to create
a reverse relationship (something that would require an
EntityResolver to be present). We just create it one-way. So a user
can build an object graph of an arbitrary size without a context and
then at some point do one of the following with it:
* "ObjectContext.registerNewObject(..)" (existing; equivalent to JPA
"persist" method)
* "ObjectContext.aNewMethod(..)" (does not exist yet; equivalent to
JPA "merge" method and somewhat of an equivalent of a "localObject"
method).
Both would traverse a graph of transient objects (since they are not
persistent yet, the graph is assumed to be finite and will not
trigger sucking the entire DB in memory), attach them to the context
and connect missing reverse relationships. The difference is that in
the first case we'd assume that the objects are not yet persistent,
while in a second case we'd attempt to match them against existing DB
rows. (a typical use of a second case would be receiving XML-
serialized stream of objects corresponding to the existing data).
But you have something different in mind? Could you elaborate on the
use cases?
Thanks
Andrus
On Oct 16, 2007, at 1:25 AM, Kevin Menard wrote:
> 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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