CoreData insertNewObjectForEntityForName and executeFetchRequest return nil

Question

I'll try to expose my problem, because is a bit complex. I use Core Data and I have a problem with the data stored. When I use this code:

NSManagedObjectContext *context = [appDelegate managedObjectContext];
NSFetchRequest *request = [[NSFetchRequest alloc] initWithEntityName:@"ItemMessage"];
NSError *error = nil;
NSArray *results = [context executeFetchRequest:request error:&error];
NSMutableArray *values = [[NSMutableArray alloc] init];

if (error == nil) {
    for (int i = 0; i<results.count; i++) {
        NSLog(@"results %@",[results objectAtIndex:i]);
        ItemMessage *itemMessage = [results objectAtIndex:i];
        [values addObject:itemMessage];
    }

ecc. the problem is that the value printed by NSLog is correct (the "results" contains something) but the itemMessage contains always 0 key/value pairs (it seems empty). To understand what is the problem I went back and saw that in insertNewObjectForEntityForName I have also this problem, this is the code that I used when I save the messages data in Core Data:

for (id key in objectMessage) {
    ItemMessage *itemmessage = [[ItemMessage alloc] init];
    itemmessage.itemMessageId = [key objectForKey:@"itemMessageId"];
    itemmessage.message = [key objectForKey:@"message"];
    itemmessage.sender = [key objectForKey:@"sender"];
    itemmessage.users = [key objectForKey:@"users"];

    NSManagedObjectContext *context = [appDelegate managedObjectContext];
    NSManagedObject *newMessage;
    newMessage = [NSEntityDescription insertNewObjectForEntityForName:@"ItemMessage" inManagedObjectContext:context];
    [newMessage setValue: itemmessage.itemMessageId forKey:@"itemMessageId"];
    [newMessage setValue: itemmessage.message forKey:@"message"];
    [newMessage setValue: itemmessage.sender forKey:@"sender"];
    [newMessage setValue: itemmessage.users forKey:@"users"];
    [context save:&error];
    if (error != nil) {
        NSLog(@"Coredata error");
    }

The problem is that newMessage after the insertNewObjectForEntityForName and the setValue contains also 0 key/value pairs. Can you help me?

Answer 1

You don't seem to insert the new managed objects correctly into the context.
It should be:

for (id key in objectMessage) {
    NSManagedObjectContext *context = [appDelegate managedObjectContext];
    ItemMessage *itemmessage = (ItemMessage*)[NSEntityDescription insertNewObjectForEntityForName:@"ItemMessage" 
                                                                           inManagedObjectContext:context];
    itemmessage.itemMessageId = [key objectForKey:@"itemMessageId"];
    itemmessage.message = [key objectForKey:@"message"];
    itemmessage.sender = [key objectForKey:@"sender"];
    itemmessage.users = [key objectForKey:@"users"];
}
//save your inserts

To create a class file for your managed objects you could:

Go to your model file (xcdatamodeld) ->
select an entity ->
from the menu select:
Editor-> Create NSManagedObjectSubclass -> select the entities your like class files for.

Now you will have managed objects you could access with ease (NSManagedObject subclass) and benefit from CoreData features.

Answer 2

When you insert to manage object contest you have to call save: method, also the saving method should looks something like that:

newMessage = [NSEntityDescription insertNewObjectForEntityForName:@"ItemMessage" inManagedObjectContext:context];
//  2
  newMessage.property1 = self.firstNameTextfield.text;
  newMessage.property2 = self.lastNameTextfield.text;

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