Django Piston -> HTTPRiot -> Core Data parser/converter. Was using this approach prior to moving over to RestKit framework (http://restkit.org/).

HRObject.m

//
//  HRObject.m
//
//  Created by Chad Podoski on 11/23/09.
//  Copyright 2009 Shacked Software. All rights reserved.
//

#import "HRObject.h"

#import "AppDelegate_Shared.h"

@interface HRObject (Private)

+ (void)populateObject:(id)object withDictionary:(NSDictionary *)dict;
+ (id)managedObjectForEntity:(NSString *)entity uuid:(NSString *)uuid;

@end


@implementation HRObject

+ (void)initialize {
    [self setDelegate:self];
    [self setBaseURL:[NSURL URLWithString:SSBaseApiUrl]];
	[self setFormat:HRDataFormatJSON];
    
    //NSDictionary *params = [NSDictionary dictionaryWithObject:@"1234567" forKey:@"api_key"];
    //[self setDefaultParams:params];
}

/**
 * Called when the resource was succeffully fetched and encoded
 *
 * @param connection The connection object for the current request
 * @param resource The converted objc representation of the response data returned by the server.
 * @param object Any custom object you passed in while making the request.
 */
+ (void)restConnection:(NSURLConnection *)connection didReturnResource:(id)resource object:(id)object {
	
	/* Converts from the HTTPRiot model class to Core Data Model class and CRUD in managed object context */
    NSMutableSet *responseSet = [self convertToCoreData:resource];
	
	if (!IsEmpty(object) && [object respondsToSelector:@selector(httpRiotResponseReceived:)]) {
		[object performSelector:@selector(httpRiotResponseReceived:) withObject:responseSet];
	}
}

/**
 * Called when the connection receieves any type of response
 *
 * @param connection The connection object for the current request
 * @param response The response object returned by the server.
 * @param object Any custom object you passed in while making the request.
 */
+ (void)restConnection:(NSURLConnection *)connection didReceiveResponse:(NSHTTPURLResponse *)response object:(id)object {
    NSLog(@"RESPONSE STATUS WAS:%i", [response statusCode]);
}

/**
 * Called when the connection fails in situations where the server is not available, etc.
 *
 * @param connection The connection object for the current request
 * @param error The error returned by the connection.
 * @param object Any custom object you passed in while making the request.
 */
+ (void)restConnection:(NSURLConnection *)connection didFailWithError:(NSError *)error object:(id)object {
    AlertWithMessage([error localizedDescription]);
}

/**
 * Called when the connection receieves a statusCode that isn't a success code.
 *
 * @param connection The connection object for the current request
 * @param error The error returned by the connection.
 * @param response The response object returned by the server.
 * @param object Any custom object you passed in while making the request.
 */
+ (void)restConnection:(NSURLConnection *)connection didReceiveError:(NSError *)error response:(NSHTTPURLResponse *)response object:(id)object {
    AlertWithMessage([error localizedDescription]);
}

/**
 * Called when the HRFormatter recieved an error parsing the response data.
 *
 * @param connection The connection object for the current request
 * @param error The parser error returned by the formatter.
 * @param body A string representation of the response body returned by the server.
 * @param object Any custom object you passed in while making the request.
 */
+ (void)restConnection:(NSURLConnection *)connection didReceiveParseError:(NSError *)error responseBody:(NSString *)body object:(id)object {
    AlertWithMessage([error localizedDescription]);
}


#pragma mark - Core Data Methods
+ (NSMutableSet *)convertToCoreData:(id)resource {
	
	NSMutableSet *result;

	/* Determine if we are dealing with a single object or an array of objects */
	if ([resource isKindOfClass:[NSDictionary class]]) {
		
		NSDictionary *dict = (NSDictionary *)resource;
		NSString *entity = [dict valueForKey:SSEntityKey];
		NSString *uuid = [dict valueForKey:SSUuidKey];
		
		/* Make sure object an entity name and an uuid */
		if (!IsEmpty(entity) && !IsEmpty(uuid)) {
		
			result = [NSMutableSet setWithCapacity:1];
		
			/* Class name needs to be prefixed with class prefix for this project */
			id object = [self managedObjectForEntity:entity uuid:uuid];
			
//			NSLog(@"******************************");
//			NSLog(@"BEFORE %@", object);
			[self populateObject:object withDictionary:resource];
//			NSLog(@"------------------------------");
//			NSLog(@"AFTER %@", object);
			
			[result addObject:object];
		}
		
	}else if ([resource isKindOfClass:[NSArray class]]) {
		
		result = [NSMutableSet setWithCapacity:[(NSArray *)resource count]];
		
		for (id object in resource) {
			[result unionSet:[self convertToCoreData:object]];
		}
	}
	
	return result;
}

+ (void)populateObject:(id)object withDictionary:(NSDictionary *)dict {
	
	NSArray *keys = [dict allKeys];
	
	for (NSString *key in keys) {
	
		id value = [dict valueForKey:key];
		key = [key capitalizedString];
		key = [key stringByReplacingOccurrencesOfString:@"_" withString:@""];
        
		/* An NSArray value indicates that this is a relation */
		if ([value isKindOfClass:[NSArray class]]) {
			
			/* Recursive build the relation */
			NSMutableSet *relationSet = [self convertToCoreData:value];
			
			/* Add relation to object */
			SEL selector = NSSelectorFromString([NSString stringWithFormat:@"add%@:", key]);
			
			if ([object respondsToSelector:selector]) {
				[object performSelector:selector withObject:relationSet];
			}
			
		}
		
		/* A NSDictionary indicate a 1-to-1 relationship from Django Piston */
		else if ([value isKindOfClass:[NSDictionary class]]) {
			
			NSMutableSet *relationSet = [self convertToCoreData:value];
			
			if (!IsEmpty(relationSet) && [relationSet count] == 1) {
				
				/* Add object to model */
				SEL selector = NSSelectorFromString([NSString stringWithFormat:@"set%@:", key]);
				
				if ([object respondsToSelector:selector]) {
					[object performSelector:selector withObject:[relationSet anyObject]];
				}
			}
		}
		
		/* It is a simple attribute */
		else {
			
			SEL setter = NSSelectorFromString([NSString stringWithFormat:@"set%@:", key]);
			
			if ([object respondsToSelector:setter]) {
				
				if (!IsEmpty(value) && ![value isEqual:[NSNull null]]) {
                    
                    if ([value isKindOfClass:[NSString class]]) {
                        
                        if (![value isEqualToString:@"<null>"]) {
                            /* Just lowercase the first character */
                            key = [key stringByReplacingCharactersInRange:NSMakeRange(0,1) withString:[[key substringToIndex:1] lowercaseString]];
                            
                            SEL getter = NSSelectorFromString(key);
                            
                            if ([object respondsToSelector:getter]) {
                                
                                NSManagedObject *mObject = (NSManagedObject*)object;
                                NSPropertyDescription *propDesc = [[[[mObject class] entityDescription] 
                                                                    propertiesByName] objectForKey:key];
                                
                                
                                
                                if ([propDesc isMemberOfClass:[NSAttributeDescription class]]) {
                                    
                                    NSAttributeDescription *attribDesc = (NSAttributeDescription*)propDesc;
                                    
                                    if ([attribDesc attributeType] == NSDateAttributeType) {
                                        NSDate *date = [NSDate dateFromString:value];
                                        if (!IsEmpty(date)) {
                                            [object performSelector:setter withObject:date];
                                        }
                                    }
                                    else if ([attribDesc attributeType] == NSDecimalAttributeType) {
                                        NSDecimalNumber *number = [NSDecimalNumber decimalNumberWithString:value];
                                        if (!IsEmpty(number)) {
                                            
                                            [object performSelector:setter withObject:number];
                                        }
                                    }
                                    else {
                                        [object performSelector:setter withObject:value];
                                    }
                                }
                            }
                        }
                    }
                    else {
                        [object performSelector:setter withObject:value];
                    }
				}
			}
		}
	}
}

+ (id)managedObjectForEntity:(NSString *)entity uuid:(NSString *)uuid {

	id object;
	
	NSManagedObjectContext *moc = [(AppDelegate_Shared*)[[UIApplication sharedApplication] delegate] managedObjectContext];
	NSFetchRequest *request = [[[NSFetchRequest alloc] init] autorelease];
	[request setEntity:[NSEntityDescription entityForName:entity inManagedObjectContext:moc]];
	[request setPredicate:[NSPredicate predicateWithFormat:@"uuid = %@", uuid]];
	
	NSError *error = nil;
	NSArray *results = [moc executeFetchRequest:request error:&error];
	
	if (error) {
		NSLog(@"%@", error);
	}
	else if (!IsEmpty(results)) {
		object = [results objectAtIndex:0];
	}
	else {
		object = [NSEntityDescription insertNewObjectForEntityForName:entity inManagedObjectContext:moc];;
	}
	
	return object;			
	
}

@end