How to use NSManagedObject without adding it to db?

Question

I'm having class that inherits NSManagedObject that was generated using my db model:

// .h
@interface Sketch : NSManagedObject

@property (nonatomic, retain) NSDate * added;
@property (nonatomic, retain) NSString * board;
@property (nonatomic, retain) NSString * filepath;
@property (nonatomic, retain) NSString * title;
@property (nonatomic, retain) NSString * filename;

@end

// .m
@implementation Sketch

@dynamic added;
@dynamic board;
@dynamic filepath;
@dynamic title;
@dynamic filename;

@end

I'm using that class instances in UITableView. Now I need to add some instances that are not stored in db (just to show them in the list):

Sketch sketch = [[Sketch alloc] init];

But when trying to set instance properties

sketch.title = @"test title";

I'm getting exception:

-[Sketch setTitle:]: unrecognized selector sent to instance 0x7ff112c13e30

Does it mean I have to create instance by adding them to Managed Context only (even if I'm not going to store them)?

[NSEntityDescription insertNewObjectForEntityForName:SKETCH_ENTITY
                                     inManagedObjectContext:context];

Answer 1

No, you can create instances of NSManagedObject subclasses and add them to the managed object context later (while I'd suggest not do do so). You have some issue with your Sketch object, not with NSManagedObject and NSManagedObjectContext.

The only thing is that you should create it like this:

NSManagedObjectContext *moc = ... // your managed object context
NSEntityDescription *entity = [NSEntityDescription entityForName:@"Sketch"
                                          inManagedObjectContext:moc];
// note nil for context
Sketch *unassociatedObject =
    (Sketch *)[[NSManagedObject alloc] initWithEntity:entity
                       insertIntoManagedObjectContext:nil];

For more details see this answer.