Re: Baffling problem with foreign key
Re: Baffling problem with foreign key
- Subject: Re: Baffling problem with foreign key
- From: Gary Teter <email@hidden>
- Date: Thu, 4 Dec 2008 15:33:41 -0800
On Dec 4, 2008, at 2:45 PM, Chuck Hill wrote:
On Dec 4, 2008, at 2:21 PM, Gary Teter wrote:
Ahh, the joys of using a debugger. Here's the sequence of events
that I think is fouling up EOF:
Set serviceRate on OrderObject.
OrderObject calls willChange().
EOEditingContext does its objectWillChange(). During this method,
it sees that OrderObject's __lastSnapshot is null, so it sets that
to the OrderObject's current snapshot. (This contains the new
serviceRate.) Then it checks to see if the __originalSnapshot is
null, and it is, so it sets the __originalSnapshot to the
__lastSnapshot.
OK, that is the problem but I suspect this is after the cause. The
cause is whatever is discarding the __originalSnapshot. I can't
see why that would be null except for a new object.
I think that actually gets nulled out every time you save a change to
an object. It gets refilled during EOEditingContext's
objectWillChange and committedSnapshotForObject (and possibly
elsewhere I guess).
Maybe the EC state is getting borked and it starts to think this is
a new object.
Is revert() or undo() getting called on this ec? You mentioned
before removing an order item. Is that doing an ec.deleteObject()?
OrderObject has ownsDestination on its orderLineItems relationship,
so deleting a line item is just
removeObjectFromBothSidesOfRelationshipWithKey. I've reduced the
number of steps to cause the problem and now believe that I may be
triggering a bug in EOF's implementation of deleting orphaned objects
in such relationships.
I have a feeling there may be a defect in some combination of
those. Are there any delete rules that might be getting fired? Is
saveChanges() ever called (even if it fails with a validation error)?
This code saves changes All. The. Frigging. Time. I inherited it from
someone whose approach then to solving problems was to add another
call to saveChanges because, I guess, hey, it couldn't hurt, right?....
I believe this is the point at which EOF is borked -- comparing
the original snapshot and the last snapshot will yield no change
to the serviceRate relationship, so it's never updated in the
database.
Of course, this raises the question: Why is __originalSnapshot
null at this point? During a non-borked save, __lastSnapshot is
null but __originalSnapshot has already been filled with the
correct values.
I wish these methods weren't private so I could set breakpoints on
them....
private methods are an annoying developer conceit. At one point, I
resorted to decompiling and re-implementing EOEditingContext so
that I could debug a very similar problem.
I also hate methods marked "final".
Chuck
On Dec 3, 2008, at 3:39 PM, Gary Teter wrote:
On Dec 3, 2008, at 12:08 PM, Chuck Hill wrote:
Hi Gary,
Nice to see you around!
I've been lurking. :-)
On Dec 3, 2008, at 10:11 AM, Gary Teter wrote:
This one has had me scratching my head for two days. I have a
situation where I can set a to-one relationship on an eo but
the change to the foreign key is not reflected in the SQL
emitted during saveChanges.
This code works properly in most circumstances, but a customer
found that if you follow a very specific series of otherwise
completely ordinary actions that work perfectly well in any
other context, then at some point EOF gets confused. The object
graph no longer matches what's in the database.
I think the particular sequence of user actions is a red
herring, but they're along the lines of, "Add two products to
your cart. Proceed to checkout. Change shipping address. Remove
item from cart. Proceed to checkout. Change shipping
method." (This is the relationship that becomes borked.) We
process tons of orders every week with this code, so it's not
like the shipping method relationship is simply broken.
It is possible that an EO is getting removed from an EC and then
altered?
No objects are being deleted during these operations as far as I
can tell. Only two new objects (order line items) are being
inserted. The relationship in question is actually two
relationships, between three objects that are already in the
database. OrderObject <<---> DeliveryServiceClass, and
OrderObject <<---> ServiceRate. (Setting the ServiceRate for an
order also sets the DeliveryServiceClass.)
This may produce the same bugs that modifying an EO before it
has been inserted cause. I take it you can reproduce this
yourself and that no back tracking or multiple browser windows
are involved?
Yes. It's like doing a combo move in a video game. If I vary any
of the steps it does not fail. If I perform the sequence
precisely right it fails. Grr.
It is possible that an EO is getting removed from an EC and re-
inserted?
It is possible that these steps are resulting in an EO in an
unlocked EC getting altered?
Sadly, this is all in the session's default editing context. I'm
pretty sure it's not a locking/unlocking issue because I'd have
noticed that long before now.
I'm pretty sure I've eliminated threading and remote
synchronization as possible culprits (can still duplicate with
those turned off). This is WebObjects 5.3.3.
The following are true when it's failing:
I can repeatedly change the destination object, save changes,
and not see the proper update statement.
If the object has other changes that need saving, an update
statement gets properly generated for those attributes, but not
the changed foreign key.
Logging statements in the setBorkedRelationship(OtherObject
value) method show that the relationship is being set properly.
Overriding snapshot() and changesFromSnapshot() show that
during saveChanges the current snapshot includes the new value,
and so does the snapshot being compared. This is definitely not
normal, usually the snapshot being compared will include the
old value, so that changesFromSnapshot() will indicate the
foreign key needs to be updated.
That really does sound like what I was seeing when modifying an
EO that was not yet in an EC.
I'm reasonably sure that our code doesn't commit that particular
sin anywhere.
Is the only updated data in the snapshot being compared the FK?
That's what it >should< be, but what I'm seeing is that the
snapshot being compared is identical to the object's current
snapshot, so EOF thinks there are no changes that need to be
saved. Even though if I step through with a debugger the object
is clearly on the list of objects which need to be updated.
Now I must be seeing things. If other attributes on the object
are also being changed, they are not showing up as different in
the snapshot being compared. Yet the update statement issued
properly includes those attributes (but not the two FKs).
The other updated values only appear in snapshot()? Is this
key getting propagated from some other object? A mutable value
can also do this, but I can't see you using an mutable FK.
Gah. No mutable FK's. The relationships are modeled like so:
OrderObject:
{
allowsNull = Y;
columnName = serviceRateID;
name = serviceRateID;
prototypeName = id;
},
{
allowsNull = Y;
columnName = deliveryServiceClassID;
name = deliveryServiceClassID;
prototypeName = id;
userInfo = {"_EntityModeler" = {generateSource =
NO; }; };
}
...
{
destination = ServiceRate;
isToMany = N;
joinSemantic = EOInnerJoin;
joins = ({destinationAttribute = id; sourceAttribute =
serviceRateID; });
name = serviceRate;
},
{
destination = DeliveryServiceClass;
isToMany = N;
joinSemantic = EOInnerJoin;
joins = ({destinationAttribute = id; sourceAttribute =
deliveryServiceClassID; });
name = deliveryServiceClass;
},
The back-pointing relationships from ServiceRate and
DeliveryServiceClass are modeled but unmarked as class properties.
Hmmmmm.... Now that's odd. What's that generateSource = NO doing
in there? Ah well, taking it out and doing a full clean/build
didn't help.
If the foreign key is marked as "use for locking", the update
SQL will properly include the >old< foreign key in the where
clause, but the foreign key column is not in the list of
columns being updated. This is interesting to me because one of
my theories was that somehow the EODatabase's snapshot was
getting updated behind my back somehow to reflect the new
value. But this indicates to me that the committed snapshot
does in fact have the correct (old) value.
Wow. That is odd. That makes it seem as if the problem is
local to that EC.
If I modify a different to-one relationship and save changes,
that change gets reflected properly, and I can then
successfully change the previously borked relationship.
Maybe saving that change resets the internal EC state?
That's my guess, but I don't want to add some hacky thing like
"change this extra relationship for no good reason other than
once in awhile EOF gets confuzzled." The fact that my object
graph can get out of sync with the database seriously scares the
hell out of me. What if it's not just this one specific set of
circumstances? Bad enough that it leads to an order being sent
via the wrong delivery service and ticks off a customer. What if
it's happening when I'm processing credit card transactions or
something? Argh.
I must be breaking some EOF commandment but damned if I can
figure out which one.... Suggestions, hints, theories greatly
appreciated.
That is the best I can do at this point.
--
The Ent visualization project:
http://wirehose.com/research/entvisualization
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