Fetching Arrays From Core Data - objective-c

Question

I've set up a model with a few string fields and a few array fields. The model saves as such:
- (void)saveLevel:(NSString*)level traps:(NSArray*)traps whirls:(NSArray*)whirls accels:(NSArray*)accels walls:(NSArray*)walls dest:(NSString*)dest jupiter:(NSString*)jupiter rating:(NSNumber*)pRating;
{
if (m_pMOC == nil)
// Retrieves the managed object context
NSManagedObject* pNewLevel = [NSEntityDescription insertNewObjectForEntityForName:#"Level" inManagedObjectContext:m_pMOC];
NSDate* pDate = [NSDate date];
// Must have these four attributes
[pNewLevel setValue:level forKey:#"Level_ID"];
[pNewLevel setValue:jupiter forKey:#"Ball"];
[pNewLevel setValue:pDate forKey:#"Creation_Date"];
[pNewLevel setValue:dest forKey:#"Destination"];
[pNewLevel setValue:pRating forKey:#"Rating"];
// Optional attributes
if ([traps count] != 0)
[pNewLevel setValue:traps forKey:#"Traps"];
if ([whirls count] != 0)
[pNewLevel setValue:whirls forKey:#"Whirls"];
if ([accels count] != 0)
[pNewLevel setValue:accels forKey:#"Accelerators"];
if ([walls count] != 0)
[pNewLevel setValue:walls forKey:#"Walls"];
NSError* pError;
if (![m_pMOC save: &pError])
// etc...
}
I have a DataManager class which handles the fetching/saving from/to Core Data, and inside of the Manager I've verified that when I fetch an entity, the arrays are fetched, but when it's returned to the class that's calling it, the arrays are nil (the strings, on the other hand, arrive just fine). Here's the fetch code and the code that parses out the returned value from the fetch:
- (NSArray*) getLevelWithID:(NSString*)level
{
NSEntityDescription *pEntityDescription = [NSEntityDescription entityForName:#"Level" inManagedObjectContext:m_pMOC];
NSFetchRequest *pRequest = [[[NSFetchRequest alloc] init] autorelease];
[pRequest setEntity:pEntityDescription];
NSPredicate* pPredicate = [NSPredicate predicateWithFormat:#"Level_ID == %#", level];
[pRequest setPredicate: pPredicate];
NSError* pError;
NSArray* pLevels = [m_pMOC executeFetchRequest:pRequest error:&pError];
if (pLevels == nil)
{
NSLog(#"Could not find the level with Level_ID = %#",level);
abort();
}
NSArray* pAccels = [pLevels valueForKey:#"Accelerators"];
NSArray* pTraps = [pLevels valueForKey:#"Traps"];
NSArray* pWalls = [pLevels valueForKey:#"Walls"];
NSArray* pWhirls = [pLevels valueForKey:#"Whirls"];
return pLevels;
}
I set a break point on the last four Arrays, and they have objects in them, but in the function that retrieves them (shown below), they are nil.
- (void) initLevel:(NSArray*)pLevelObjects
{
Level* pLevel = [pLevelObjects objectAtIndex:0];
NSString* pJupiter = pLevel.Ball;
NSString* pDest = pLevel.Destination;
NSArray* pAccels = [pLevel valueForKey:#"Accelerators"];
NSArray* pTraps = [pLevel valueForKey:#"Traps"];
NSArray* pWalls = [pLevel valueForKey:#"Walls"];
NSArray* pWhirls = [pLevel valueForKey:#"Whirls"];
... (other initialization of the level) ...
}
I'm perplexed by this. The string values are there, but the arrays are not. I tried using the dot notation originally (NSArray* pAccels = pLevel.Accelerators, etc), but with the same result.
Ideas?
EDIT: initLevel is called from the view controller:
- (void) startGameWithLevelID:(NSString*)pLevelID
{
NSLog(#"MyViewController - beginGameWithLevelID");
NSArray* pLevel = [[DataManager getDataManager] getLevelWithID:pLevelID];
if (pLevel == nil || [pLevel count] == 0)
{
NSLog(#"Didn't retrieve any level with id %#", pLevelID);
abort();
}
else
{
CGRect newFrame = makeScreen([UIScreen mainScreen].applicationFrame);
GameBoard* pGB = [[GameBoard alloc] initWithFrame: newFrame];
pGB.m_pMyVC = self;
[pGB initLevel: pLevel];
[self.view addSubview: (UIView*)pGB];
[pGB release];
}
}
EDIT: I've newly re-factored the arrays into separate entities with a to-one relationship with the Level; the level has a to-many relationship with these entities. I also changed the names to better adhere to conventions: all attributes and relationships now start with a lowercased letter. I've prefixed the attributes and relationships with a_ and r_ respectively. I'm getting an error when saving "-[NSCFString _isKindOfEntity:]: unrecognized selector sent to instance 0x4d83120". It isn't terribly helpful but I'll let you know when I find the issue. For now, the code for saving looks as such:
- (void)saveLevel:(NSString*)level traps:(NSArray*)traps whirls:(NSArray*)whirls accels:(NSArray*)accels walls:(NSArray*)walls dest:(NSString*)dest jupiter:(NSString*)jupiter rating:(NSNumber*)pRating;
{
if (m_pMOC == nil)
{ // Code to get the managed object context from the delegate
}
NSManagedObject* pNewLevel = [NSEntityDescription insertNewObjectForEntityForName:#"Level" inManagedObjectContext:m_pMOC];
NSManagedObject* pNewBall = [NSEntityDescription insertNewObjectForEntityForName:#"Ball" inManagedObjectContext:m_pMOC];
[pNewBall setValue:jupiter forKey:#"a_Bounds"];
NSManagedObject* pNewDest = [NSEntityDescription insertNewObjectForEntityForName:#"Dest" inManagedObjectContext:m_pMOC];
[pNewDest setValue:dest forKey:#"a_Bounds"];
NSDate* pDate = [NSDate date];
// Must have these four attributes
[pNewLevel setValue:level forKey:#"a_Level_ID"];
[pNewLevel setValue:pDate forKey:#"a_Creation_Date"];
[pNewLevel setValue:pRating forKey:#"a_Rating"];
[pNewLevel setValue:#"Bob Dole" forKey:#"a_CreatedBy"];
[pNewLevel setValue:pNewBall forKey:#"r_Ball"];
[pNewLevel setValue:pNewDest forKey:#"r_Dest"];
// Optional attributes
if ([traps count] != 0)
[[pNewLevel mutableSetValueForKey: #"r_Trap"] addObjectsFromArray: traps];
if ([whirls count] != 0)
[[pNewLevel mutableSetValueForKey: #"r_Whirl"] addObjectsFromArray: whirls];
if ([accels count] != 0)
[[pNewLevel mutableSetValueForKey: #"r_Accel"] addObjectsFromArray: accels];
if ([walls count] != 0)
[[pNewLevel mutableSetValueForKey: #"r_Wall"] addObjectsFromArray: walls];
NSError* pError;
if (![m_pMOC save: &pError])
{ // Error saving
}
else
{ // Successfully saved
}
}
Again, thanks for the help, sorry for the lengthy post but I'm hoping this will help other Core Data newbies (like me) learn something.
FINAL EDIT: Ok, so after refactoring, I finally found a solution to save the arrays as objects (though whether it's the most efficient way I'm not sure). Below is just the code for adding a bunch of "Traps". The array passed into the function is the bounds of the Trap, which will be stored. The relationship is then formed with the Level (note it doesn't have to be set by both the Trap and the Level, just one of them, since Core Data takes care of the other way).
if ([traps count] != 0)
{
for (NSString* pBounds in traps)
{
NSManagedObject* pNewTrap = [NSEntityDescription insertNewObjectForEntityForName:#"Trap" inManagedObjectContext:m_pMOC];
[pNewTrap setValue:pBounds forKey:#"a_Bounds"];
[pNewTrap setValue:pNewLevel forKey:#"r_Level"];
}
}

Answer 1

Possible Array Misuse
In your first snippet, you are calling valueForKey: on an NSArray, and the result is an array, because that code iterates over pLevels, passes your valueForKey: argument to each object inside and combines the values.
It is how NSArray implements Key-Value Coding.
In your second piece of code, where you first fetch one pLevel from the array, you perform your valueForKey: selector on an entity, not on an array. So the result is what's in that particular object.
Probably some of your entities have those fields, so a combined result from all of those has some values, but some particular ones do not.
Possible Core Data Misuse
You say that you keep an array in your Core Data entity. How do you do that? There isn't a special type to hold an array, so normally it has to be stored as binary data with performing encoding using [NSKeyedArchiver archivedDataWithRootObject:array]. It might be that your storing and fetching is incorrect because you're trying to store arrays in non-array fields.
Do you also experience any crashes?