JSON into Core Data to be displayed via UITableVewController

Answers (3)

Reputation: 64428

The immediate source of your error is that you are sending the save:error: message to a non-existant managed object context. In turn, the managed object context doesn't exist because you never created or inserted the managed object newNote into a context to begin with.

I get the impression that you haven't quite grasp that Core Data is built around objects instead of SQL tables, columns, rows etc. Every piece of data in Core Data is stored in one or more objects or the relationships between. All those objects and relationships create and object graph and it is that graph that Core Data really manages.

In your case, every JSON message decomposes into to numerous dictionaries, each one containing the data for one note. You need one managed object configured with the Event entity, to store each note.

If you start with one of the Core Data using templates in Xcode, you will see that you get the managed object context, managed object model, persistent store coordinator and persistent stores all setup for you and assigned to attributes of the app delegate. In the following code I will use the attributes and names from the template.

You need something like this (I didn't compile this so watch for typos):

-(void)pullNotes{
    //first, get the application delegate that manages the Core Data stack
    // since the app itself is a singleton, we just ask it for its delegate
    MyAppDelegateClass *appDelegate=(MyAppDelegateClass *) [[UIApplication sharedApplication] delegate];

    // ...all the JSON pulling code

    NSManagedObject *newNote; 
    NSArray *notes = [object valueForKey:@"result"];
    for (NSDictionary *singleNote in notes){
        // create another concrete instance of NSManagedObject 
        // for each logical note from JSON and insert it into
        // the appDelegate's managedObjectContext attribute
        newNote=[NSEntityDescription insertNewObjectForEntityForName:@"Event" 
                                              inManagedObjectContext:appDelegate.managedObjectContext_];
        // Assign data from the JSON note to the attributes of this specific 
        // NSManagedObject instance. 
        [newNote setValue:[singleNote objectForKey:@"note content"] forKey:@"noteContent"];
        // ... repeat for all other attributes you wish to have in the data 
    } 

    // only save after you've created all the notes. It's to slow to 
    // save after every note.   
    NSError *error = nil;
    if (![appDelegate.managedObjectContext_ save:&error]) {
        NSLog(@"Unresolved error %@, %@", error, [error userInfo]);
        abort();
    }
}

Key points:

Each piece of modeled data should have its own instance of a NSManagedObject.
All managed object instance must be configured with an entity and inserted into a managed object context to be managed or saved.
The context and other parts of the Core Data stack usually reside in the app delegate and you usually have only one stack at a time (for most apps.) Use the UIApplication singleton to access the app delegate from anywhere in the app.

Update:

However, it adds entities regardless of whether or not they were there before. What I want it to do is to find that if a note is already there, update it. If not, create a new one.

To check for a pre-existing note, you need to attempt to fetch existing notes matching the new data. Here's simple implementation (uncompiled):

// call this outside the loop so you 
// only have to call it once.
NSEntityDescription *noteEntity=[NSEntityDescription entityForName:@"Event" inManagedObjectContext:appDelegate.managedObjectContext];
NSFetchRequest *noteFetch;
NSArray *fetchedNotes;

// inside the for loop for each new note
noteFetch=[[NSFetchRequest alloc] init];
[noteFetch setEntity:noteEntity];
NSPredicate *pred=[NSPredicate predicateWithFormat:@"noteID==%@",[singleNote objectForKey:@"note id"]];
[noteFetch setPredicate:pred];

NSError *fetchError=nil;
fetchedNotes=[appDelegate.managedObjectContext_ performFetch:&fetchError];
if (fetchError!=nil) {
    NSLog(@"pullNotes fetchError=%@,details=%@",fetchError,fetchError.userInfo);
}
if ([fetchedNotes count]==0) {
    // create new note
}
[noteFetch release];
// end loop

You can form a predicate using the IN test that would test against an array of all the new ``noteID values and do just one fetch but that is a little more complex. Unless you are processing hundreds of notes at a time, this technique will look just as fast from the user's perspective.

Upvotes: 1

Mike C.

Reputation: 601

You're just calling insertNewObjectForEntityForName which always creates a new object. Use a unique value as the key, create a fetch request for that key, and if it returns nothing, then you create a new object.

 + (Note*)NoteWithKey: (NSString*)key
{
    NSDictionary *query = [NSDictionary dictionaryWithObjectsAndKeys:key,@"KEY",nil];

    NSManagedObjectContext *context = appDelegate.managedObjectContext;
    NSManagedObjectModel *model = appDelegate.managedObjectModel;
    NSFetchRequest *fetchRequest = [model fetchRequestFromTemplateWithName:@"GetKey" substitutionVariables:query];

    NSError *error;
    NSArray *results = [context executeFetchRequest:fetchRequest error:&error];
    if (results && ([results count] > 0)) {
        return [results objectAtIndex:0];
    }
    else {
        Note *newNote = [NSEntityDescription insertNewObjectForEntityForName:@"Note" inManagedObjectContext:context];
        if (nil != newNote) {
            [newNote setValue:key forKey:@"Key"];
        }

        return newNote;
    }
}

Upvotes: 1

Joris Kluivers

Reputation: 12081

Your header file ( http://cl.ly/4rz2 ) has a reference for an Event object, called newNote. But you never create an object to store in that reference. I would remove the reference in your header file, and keep it local in your JSON parsing method. Create one event for each singleNote you encounter in the json:


[..]
Event *newNote = nil;

for (NSDictionary *singleNote in notes) {
    // create a new autoreleased object
    newNote = [[[Event alloc] initWithEntity:aDescription insertIntoManagedObjectContext:aContext] autorelease];
    newNote.content = [singleNote objectForKey:@"note content"];
}

// save managedobjectcontext
[..]

Unrelated

[UIApplication alloc] is not something you normally want to do. You probably want to use something like: [UIApplication sharedApplication].n networkActivityIndicatorVisible = YES;

Upvotes: 0

JSON into Core Data to be displayed via UITableVewController

Answers (3)

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