Not getting data from Core Data

Question

I am using Core Data to store some information for my app. I have a .xcdatamodeld file containing 8 entities, and I extract them on different views.

In one of the viewControllers, I call three of them. Like this:

    AppDelegate *appDelegate = (AppDelegate *) [[UIApplication sharedApplication]delegate];

managedObjectContext = appDelegate.managedObjectContext;

NSManagedObjectContext *moc = [self managedObjectContext];


NSEntityDescription *entiAll = [NSEntityDescription entityForName:@"AllWeapons" inManagedObjectContext:moc];
NSFetchRequest *frAll = [[NSFetchRequest alloc] init];
[frAll setEntity:entiAll];
NSError *error = nil;
arrAll = [moc executeFetchRequest:frAll error:&error];
displayArray = [[NSMutableArray alloc]initWithArray:arrAll];


NSEntityDescription *entiRange = [NSEntityDescription entityForName:@"WeaponsRanged" inManagedObjectContext:moc];
NSFetchRequest *frRanged = [[NSFetchRequest alloc] init];
[frRanged setEntity:entiRange];
NSError *errorRanged = nil;
arrRange = [moc executeFetchRequest:frRanged error:&errorRanged];
NSLog(@"%i, %i", [arrRange count], [[moc executeFetchRequest:frRanged error:&errorRanged] count]);


NSEntityDescription *entiMelee = [NSEntityDescription entityForName:@"WeaponsMelee" inManagedObjectContext:moc];
NSFetchRequest *frMelee = [[NSFetchRequest alloc] init];
[frMelee setEntity:entiMelee];
NSError *errorMelee = nil;
arrMelee = [moc executeFetchRequest:frMelee error:&errorMelee];
NSLog(@"%i, %i", [arrMelee count], [[moc executeFetchRequest:frMelee error:&errorMelee] count]);

The problem is that the middle one (the one filling the arrRange-array) doesn't work..

arrAll logs out with all correct data, arrMelee logs out with all the correct data (x4 for some reason, don't know if this is related :S), but arrRange logs out as an empty array. [arrRange count]; gives me 0, even though I know there is lots of data there. I ran this code on the simulator, and found the .sqlite file, opened it in Firefox's SQLite Manager, and saw the correct data, 40 rows.

I went into the appDelegate, where I fill the CoreData when necessary, and saw that the method which downloads the data in JSON-format successfully sends it to the sqlite aswell.

Here I fill the CoreData with data from the json:

[self deleteAllObjects:@"WeaponsRanged"];
NSManagedObjectContext *context = [self managedObjectContext];

for(NSDictionary *item in jsonWeaponRanged)
{
    WeaponsRanged *wr = [NSEntityDescription insertNewObjectForEntityForName:@"WeaponsRanged" 
                    inManagedObjectContext:context];
    ///***///
    wr.recoil = [item objectForKey:@"Recoil"];
    ///***///

     NSError *error;
    if(![context save:&error])
        NSLog(@"%@", [error localizedDescription]);
}

And if I here do NSLog(@"%@ - %@", wr.recoil, [item objectForKey:@"Recoil"]); I get the correct data. (Same data on both)

So. The correct data is obviously in the core. But my NSFetchRequest or something is failing. I am pretty noob at Objective-C, so it might be my bad code-grammar striking again. I realize I should use things again etc, not creating new objects all the time.. But cmon, this is my first app.. And if that is actually the problem, I might learn. But I'm stuck.

SOMETIMES I get data, sometimes I don't. It's weird. I re-launched the app, and got data from it, and now I don't.. I haven't found a pattern yet..

Anyone? Or is there another way to request data from the entity?

Answer 1

Nevermind! It was my own damn fault (yet again..).

The problem occured before the code I presented, and it turns out the data was never in the .sqlite-file when the problem was present.

This is what I had:

I collected data from the internet through json-request. I have told the app to check the "version" of the data through the internet, and if the data is outdated, then re-download it.

First, I download all data, then I add them to their own entity in Core Data. After downloading, I clear the current Core Data entity of the downloaded data. So on the top of each add-method it said i.e [self deleteAllObjectsOfEntity:@"WeaponsRanged"];, My whole problem was that in the addMelee-method, it ALSO said [self deleteAllObjectsOfEntity:@"WeaponsRanged"]; instead of @"WeaponsMelee", thus deleting all ranged weapons, and later adding melee to the melee entity. And that also proves that the other problem I mentioned of arrMelee logging out four times as much data as it should was caused by this.

The reason it sometimes worked was that the downloading is not happening in any ordered mode. So the addRanged was sometimes called before the addMelee. If ranged comes first, it clears the arrRanged, and fills it up with correct data, and THEN melee comes, and clears it out. When melee was called first, it cleared arrRanged and filled additional data to arrMelee, and THEN ranged comes and tries to clear an empty entity, and then fills it up with correct data.

The solution was obviously to change the entity deleted when adding it, as it was the wrong one.

Sorry.... :)

Answer 2

I have some suggestions, too big for a comment.

1) after you create the WeaponsRanged, try reading them back:

for(NSDictionary *item in jsonWeaponRanged)
{
    WeaponsRanged *wr = [NSEntityDescription insertNewObjectForEntityForName:@"WeaponsRanged" 
                    inManagedObjectContext:context];
    NSLog(@"IS WR Realized? %@", wr ? @"YES" : @"NOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOO WR");
    ///***///
    wr.recoil = [item objectForKey:@"Recoil"];
    ///***///

     NSError *error;
    if(![context save:&error])
        NSLog(@"%@", [error localizedDescription]);
}

// Now lets see if we can retrieve them:
{
    NSEntityDescription *entiRange = [NSEntityDescription entityForName:@"WeaponsRanged" inManagedObjectContext:context];
    NSFetchRequest *frRanged = [[NSFetchRequest alloc] init];
    [frRanged setEntity:entiRange];
    NSError *errorRanged = nil;
    arrRange = [context executeFetchRequest:frRanged error:&errorRanged];
    NSLog(@"Wrote %i items, read back %i items", [jsonWeaponRanged count], [arrRange count] );
}

2) In the viewController reading WeaponsRanged, add an assert before the fetch on mod:

    NSLog(@"IS moc set? %@", moc ? @"YES" : @"NOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOO MOC");

EDIT:

3) Spread statements everywhere you access the MOC:

assert([NSThread isMainThread]); 

[If you haven't used asserts before google and read up on the topic. These are a powerful tool for developers to find out about potential problems way before they manifest themselves in the gui or elsewhere. They are normally compiled out for release/distribution builds.]

This will force an exception if the thread is not the main thread, and then let you track down the reason by following the stack trace.