NSManagedObject+Serialization.h

@interface NSManagedObject (Serialization)

- (NSDictionary*) toDictionary;

- (void) populateFromDictionary:(NSDictionary*)dict;

+ (NSManagedObject*) createManagedObjectFromDictionary:(NSDictionary*)dict
                                             inContext:(NSManagedObjectContext*)context;

@end

NSManagedObject+Serialization.m

#import "NSManagedObject+Serialization.h"

@implementation NSManagedObject (Serialization)

#define DATE_ATTR_PREFIX @"dAtEaTtr:"

#pragma mark -
#pragma mark Dictionary conversion methods

- (NSDictionary*) toDictionaryWithTraversalHistory:(NSMutableArray*)traversalHistory {
    NSArray* attributes = [[[self entity] attributesByName] allKeys];
    NSArray* relationships = [[[self entity] relationshipsByName] allKeys];
    NSMutableDictionary* dict = [NSMutableDictionary dictionaryWithCapacity:
                                 [attributes count] + [relationships count] + 1];

    NSMutableArray *localTraversalHistory = nil;
    
    if (traversalHistory == nil) {
        localTraversalHistory = [NSMutableArray arrayWithCapacity:[attributes count] + [relationships count] + 1];
    } else {
        localTraversalHistory = traversalHistory;
    }
    
    [localTraversalHistory addObject:self];
    
    [dict setObject:[[self class] description] forKey:@"class"];
    
    for (NSString* attr in attributes) {
        NSObject* value = [self valueForKey:attr];
        
        if (value != nil) {
            if ([value isKindOfClass:[NSDate class]]) {
                NSTimeInterval date = [(NSDate*)value timeIntervalSinceReferenceDate];
                NSString *dateAttr = [NSString stringWithFormat:@"%@%@", DATE_ATTR_PREFIX, attr];
                [dict setObject:[NSNumber numberWithDouble:date] forKey:dateAttr];
            } else {
                [dict setObject:value forKey:attr];
            }
        }
    }
    
    for (NSString* relationship in relationships) {
        NSObject* value = [self valueForKey:relationship];
        
        if ([value isKindOfClass:[NSSet class]]) {
            // To-many relationship
            
            // The core data set holds a collection of managed objects
            NSSet* relatedObjects = (NSSet*) value;
            
            // Our set holds a collection of dictionaries
            NSMutableArray* dictSet = [NSMutableArray arrayWithCapacity:[relatedObjects count]];
            
            for (NSManagedObject* relatedObject in relatedObjects) {
                if ([localTraversalHistory containsObject:relatedObject] == NO) {
                    [dictSet addObject:[relatedObject toDictionaryWithTraversalHistory:localTraversalHistory]];
                }
            }
            
            [dict setObject:[NSArray arrayWithArray:dictSet] forKey:relationship];
        }
        else if ([value isKindOfClass:[NSManagedObject class]]) {
            // To-one relationship
            
            NSManagedObject* relatedObject = (NSManagedObject*) value;
            
            if ([localTraversalHistory containsObject:relatedObject] == NO) {
                // Call toDictionary on the referenced object and put the result back into our dictionary.
                [dict setObject:[relatedObject toDictionaryWithTraversalHistory:localTraversalHistory] forKey:relationship];
            }
        }
    }
    
    if (traversalHistory == nil) {
        [localTraversalHistory removeAllObjects];
    }
    
    return dict;
}

- (NSDictionary*) toDictionary {
    return [self toDictionaryWithTraversalHistory:nil];
}

+ (id) decodedValueFrom:(id)codedValue forKey:(NSString*)key {
    if ([key hasPrefix:DATE_ATTR_PREFIX] == YES) {
        // This is a date attribute
        NSTimeInterval dateAttr = [(NSNumber*)codedValue doubleValue];
        
        return [NSDate dateWithTimeIntervalSinceReferenceDate:dateAttr];
    } else {
        // This is an attribute
        return codedValue;
    }
}

- (void) populateFromDictionary:(NSDictionary*)dict
{
    NSManagedObjectContext* context = [self managedObjectContext];
    
    for (NSString* key in dict) {
        if ([key isEqualToString:@"class"]) {
            continue;
        }
        
        NSObject* value = [dict objectForKey:key];
        
        if ([value isKindOfClass:[NSDictionary class]]) {
            // This is a to-one relationship
            NSManagedObject* relatedObject =
            [NSManagedObject createManagedObjectFromDictionary:(NSDictionary*)value
                                                           inContext:context];
            
            [self setValue:relatedObject forKey:key];
        }
        else if ([value isKindOfClass:[NSArray class]]) {
            // This is a to-many relationship
            NSArray* relatedObjectDictionaries = (NSArray*) value;
            
            // Get a proxy set that represents the relationship, and add related objects to it.
            // (Note: this is provided by Core Data)
            NSMutableSet* relatedObjects = [self mutableSetValueForKey:key];
            
            for (NSDictionary* relatedObjectDict in relatedObjectDictionaries) {
                NSManagedObject* relatedObject =
                [NSManagedObject createManagedObjectFromDictionary:relatedObjectDict
                                                               inContext:context];
                [relatedObjects addObject:relatedObject];
            }
        }
        else if (value != nil) {
            [self setValue:[NSManagedObject decodedValueFrom:value forKey:key] forKey:key];
        }
    }
}

+ (NSManagedObject*) createManagedObjectFromDictionary:(NSDictionary*)dict
                                                   inContext:(NSManagedObjectContext*)context
{
    NSString* class = [dict objectForKey:@"class"];
        
    NSManagedObject* newObject =
    (NSManagedObject*)[NSEntityDescription insertNewObjectForEntityForName:class
                                                          inManagedObjectContext:context];
    
    [newObject populateFromDictionary:dict];
    
    return newObject;
}

@end

Thank you for the category, that's much cleaner.

I've forked it to fix two issues I found when trying to create managed objects from the dictionary: my entity names didn't match my class names, so I wasn't able to restore serialized classes ("NSInternalInconsistencyException: +entityForName: could not locate an entity...")

Also the DATE_ATTR_PREFIX wasn't removed from the key before setting the value, resulting in exceptions ("NSUnknownKeyException: the entity is not key value coding-compliant for the key 'dAtEaTtr:foo'")

For resolve the problem of @nuthatch at the line 132 changed :
[self setValue:[NSManagedObject decodedValueFrom:value forKey:key] forKey:key];
to :
NSString *newKey = [key stringByReplacingOccurrencesOfString:DATE_ATTR_PREFIX withString:@""];
[self setValue:[NSManagedObject decodedValueFrom:value forKey:key] forKey:newKey];