Unrecognized selector for NSManagedObject base class

问题

I am getting an 'unrecognised selector' exception when calling a base-class method on an instance and can't see what the problem is.

I have an object called Form as follows:

#import <Foundation/Foundation.h>
#import <CoreData/CoreData.h>
#import "HPSDbBase.h"

@interface Form : HPSDbBase

@end

The base class for Form looks like this:

#import <CoreData/CoreData.h>

@interface HPSDbBase : NSManagedObject

@property (nonatomic, retain) NSString * id;
@property (nonatomic, retain) NSString * json;

-(id)getJSONElement:(NSString*)key;

@end

I then try using the Form object within a view controller method as follows:

HPSAppDelegate* appDelegate = [[UIApplication sharedApplication] delegate];

NSError* error = nil;
NSFetchRequest * request = [[NSFetchRequest alloc] init];

[request setEntity:[NSEntityDescription entityForName:@"Form" inManagedObjectContext:appDelegate.managedObjectContext]];

NSArray* arrayOfForms = [appDelegate.managedObjectContext executeFetchRequest:request error:&error];

for (int i=0;i<arrayOfForms.count;i++)
{

    Form* dbForm = [arrayOfForms objectAtIndex:i];

    NSLog(@"Form.json=%@",dbForm.json); // this works

    NSString* wwwww = (Form*)[dbForm getJSONElement:@"test"]; // exception here

}

The exception is:

-[NSManagedObject getJSONElement:]: unrecognized selector sent to instance 0x8290940

Can anyone see what I'm doing wrong?

Thanks a million!

EDIT 1

Here is the implementation for HPSDbBase:

#import "HPSDbBase.h"

@implementation HPSDbBase

@dynamic id;
@dynamic json;


-(id)getJSONElement:(NSString*)key
{
    NSData *jsonData = [[self json] dataUsingEncoding:NSUTF8StringEncoding];

    NSError *e = nil;

    id jsonObject = [NSJSONSerialization JSONObjectWithData:jsonData options: NSJSONReadingMutableContainers error: &e];
    NSDictionary *jsonDictionary = (NSDictionary *)jsonObject;

    id rc = [jsonDictionary objectForKey:key];

    return rc;
}

@end

回答1:

I tracked down the problem.

I had renamed my core-data object. I renamed everything I could see regarding the name of the core-data object, but it was obviously not enough. I deleted the core-data entity, then recreated a brand new one with the right name and everything started working.

回答2:

I had also received this error after renaming a class.

If you prefer not to delete your classes, I found that I could resolve the error by opening up my "xcdatamodeld" file, and clicking on Configurations -> Default. There, the entity's Class description was still referencing the old name. After correcting it here, the problem was resolved.

回答3:

I also received this error. It transpired that it was simply that there was a typo in the name of the attribute in the core data configuration of the entity. It was named correctly in the @dynamic statement in the class implementation so didn't raise a flag when compiled but did as soon as accessed. Because I was setting up NSManagedObject sub-Entities I was distracted from looking for the obvious. Just listing this here in case someone else is in the same boat.

来源：https://stackoverflow.com/questions/11344891/unrecognized-selector-for-nsmanagedobject-base-class

标签

objective-c

core-data

inheritance