NSManagedObject to JSON-ready dictionary and back again (Does not handle relationships). Uses https://gist.github.com/jdriscoll/6112942.

NSManagedObject+JSON.h

//
//  NSManagedObject+JSON.h
//  Rego
//
//  Created by Justin Driscoll on 7/29/13.
//  Copyright (c) 2013 Makalu Inc. All rights reserved.
//

#import <CoreData/CoreData.h>

@interface NSManagedObject (JSON)

- (NSDictionary *)dictionaryOfAttributes:(NSSet *)excludedKeys;
- (void)updateAttributesFromDictionary:(NSDictionary *)attributesDictionary;

@end

NSManagedObject+JSON.m

//
//  NSManagedObject+JSON.m
//  Rego
//
//  Created by Justin Driscoll on 7/29/13.
//  Copyright (c) 2013 Makalu Inc. All rights reserved.
//

#import "NSManagedObject+JSON.h"
#import "NSDate+Formats.h"

@implementation NSManagedObject (JSON)

- (NSDictionary *)dictionaryOfAttributes:(NSSet *)excludedKeys
{
    NSMutableDictionary *data = [NSMutableDictionary dictionary];

    NSDictionary *attributes = [[self entity] attributesByName];

    for (NSString *key in [attributes allKeys]) {
        if ([excludedKeys containsObject:key])
            continue;

        id value = [self valueForKey:key];

        if (!value)
            continue;

        if ([value isKindOfClass:[NSDate class]]) {
            value = [(NSDate *)value toJSONDate];
        }

        data[key] = value;
    }

    return data;
}

- (void)updateAttributesFromDictionary:(NSDictionary *)attributesDictionary
{
    for (NSString *key in [attributesDictionary allKeys]) {
        id value = attributesDictionary[key];

        if ([value isKindOfClass:[NSString class]]) {
            NSDate *dateValue = [NSDate fromJSONDate:value];

            if (dateValue) {
                value = dateValue;
            }
        }

        [self setValue:value forKey:key];
    }
}

@end