Why is Core Data losing one of my values? - objective-c

Question

Inside my user object I have the following code to generate a new 'session' or continue the existing session if one exists.
Strangely it will keep other properties but just loses the 'user' property... user is in a one to many relationship with session, 1 user can have many sessions. (or will do, for the following test I am simply checking for any previous session and using it if it exists)
-(void)setupSessionStuff
{
// Create new Core Data request
NSFetchRequest *request = [[NSFetchRequest alloc] init];
NSEntityDescription *entity = [NSEntityDescription entityForName:#"Session" inManagedObjectContext:[self managedObjectContext]];
[request setEntity:entity];
// Create Sort Descriptors for request
NSSortDescriptor *startTimeSort = [[NSSortDescriptor alloc] initWithKey:#"startTime" ascending:NO selector:nil];
[request setSortDescriptors:[NSArray arrayWithObjects:startTimeSort, nil]];
[startTimeSort release];
[request setFetchLimit:1]; // Only get the most recent session
// Execute request
NSError *error = nil;
NSArray *results = [[self managedObjectContext] executeFetchRequest:request error:&error];
if (results == nil) {
// Something went horribly wrong...
NSLog(#"Unresolved error %#, %#", error, [error userInfo]);
exit(-1);
}
[request release];
Session *theSession = nil;
if ([results count] == 1) {
NSLog(#"existing session");
// Use existing Session
theSession = [results objectAtIndex:0];
NSLog(#"session.user: %#", [theSession valueForKey:#"user"]); // this is always null!
} else {
NSLog(#"new session");
// Create new Sesson
theSession = (Session *)[NSEntityDescription insertNewObjectForEntityForName:#"Session" inManagedObjectContext:[self managedObjectContext]];
// Add the Session to the User
NSLog(#"before: session.user: %#", theSession.user); // null
theSession.user = self;
NSLog(#"after: session.user: %#", theSession.user); // looks good
}
...
NSLog(#"before.save: session.user: %#", theSession.user); // good
// Save everything
error = nil;
if (![[self managedObjectContext] save:&error]) {
// Something went horribly wrong...
NSLog(#"Unresolved error: %#, %#, %#", error, [error userInfo],[error localizedDescription]);
exit(-1);
}
NSLog(#"after.save: session.user: %#", theSession.user); // still there..
}
Additionally I have opened up the Core Data sqlite file and examined with SQLite Manager. It looks like the relationship has been correctly saved, as I can see the userID stored in the session table.
-
Just added this at the start of my method as another test.
NSSet *set = self.session;
for(Session *sess in set) {
NSLog(#"startTime %#", sess.startTime);
NSLog(#"user %#", sess.user);
}
Strangely enough the user is set in this case!? So set here then not set a few lines later when I do the fetch request... ?
-
In response to feedback below
Have added this code after assigning session.user = self and both return the expected output. So it does look like the problem is with the subsequent fetch.
NSLog(#"self.session: %#", self.session);
NSLog(#"self.session: %#", [self valueForKey:#"session"]);
Also I agree that accessing my session's through self.session will let me work around my issue, but it doesn't solve what is going on here.
In other places I surely won't be able to walk from one entity to the other so need to confidence the fetch is going to pull everything in correctly.

Answer 1

Firstly, I would check that the relationship is set from the other side by logging self.session. If that shows as null then you have no reciprocal relationship set in the model.
I think that your fetch is poorly constructed. You are relying on the sort descriptor to provide the last used session but you only have a fetch limit of 1. You seem to think that the fetch will find all existing Session objects, sort them and then return the first one. However, sorts execute last so the fetch will find a single session object (because of the fetch limit) and will then apply the sort to that single object. This makes it likely that you will be dealing with a more or less random Session object.
You probably don't need a fetch at all because you are only interested in the Session objects in a relationship with the User object which is self. If you already have the object in one side of a relationship, you don't need to fetch, you just need to walk the relationship. All you really need to do is:
if ([self.session count]==0){
//...create new session and set relationship
}else{
//... find latest session
}
I think your problem with this particular chunk of code is in the reporting. When you get a return value, you use the self.user notation but where you get a null return you use valueForKey:.
While in theory both return the same value, they don't have to because they don't use the same mechanism to return the value. The self-dot notation calls an accessor method while valueForKey: may or may not depending on the specifics of a class' implementation. I would test if the self.session returns a value where valueForKey: does not.

Answer 2

Well I found the problem and solved my issue...
After examining the memory address of my session entity I noticing that it was changing between runs. Investigating further I discovered that where I had been testing some code earlier in another class, creating a new session entity, but not saving it, well, it was being saved after all - when I issued the save in my code above!