ExtendedManagedObject.h

typedef BOOL (^BlockingRelationship)(id,NSString*);

@interface NSManagedObject (ExtendedManagedObject)
- (NSDictionary*) toDictionaryBlockingRelationships:(BOOL (^)(id obj,NSString *relationship))blockRelationship;
+ (NSManagedObject*) createManagedObjectFromDictionary:(NSDictionary*)dict
                                                   inContext:(NSManagedObjectContext*)context;



@end

ExtendedManagedObject.m

#import "ExtendedManagedObject.h"



@implementation NSManagedObject (ExtendedManagedObject)

#pragma mark -
#pragma mark Dictionary conversion methods

- (NSDictionary*) toDictionaryBlockingRelationships:(BlockingRelationship) blockRelationship;
{
	

		NSArray* attributes = [[[self entity] attributesByName] allKeys];
		NSArray* relationships = [[[self entity] relationshipsByName] allKeys];
		NSMutableDictionary* dict = [NSMutableDictionary dictionaryWithCapacity:
									 [attributes count] + [relationships count] + 1];
		
		[dict setObject:[[self class] description] forKey:@"class"];
		
		for (NSString* attr in attributes) {
			NSObject* value = [self valueForKey:attr];
			
			if (value != nil) {
				[dict setObject:value forKey:attr];
			}
		}


		
				NSLog(@"Looking through relationships of %@",[self class]);
			for (NSString* relationship in relationships) {
				NSLog(@"Looking for relationship of %@",relationship);
				if(!blockRelationship(self,relationship)){
					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
						NSMutableSet* dictSet = [NSMutableSet setWithCapacity:[relatedObjects count]];
							for (NSManagedObject *relatedObject in relatedObjects) {
							
								
									NSDictionary *returnedDict=[relatedObject toDictionaryBlockingRelationships:blockRelationship];
																		if(returnedDict)
									   [dictSet addObject:returnedDict];
								

							}
							
							if(dictSet)
								[dict setObject:dictSet forKey:relationship];
					}
						
					
					else if ([value isKindOfClass:[NSManagedObject class]]) {
						// To-one relationship
						NSDictionary *returnedDict=nil;
						
							NSManagedObject* relatedObject = (NSManagedObject*) value;
				
							returnedDict=[relatedObject toDictionaryBlockingRelationships:blockRelationship];
						
							if (returnedDict)
							[dict setObject:returnedDict forKey:relationship];
						
					}
				}
			}
		
	 return dict;
	
		//	traversed=YES;
	
}
+ (NSManagedObject*) createManagedObjectFromDictionary:(NSDictionary*)dict
                                                   inContext:(NSManagedObjectContext*)context
{
    NSString* class = [dict objectForKey:@"class"];
    NSManagedObject* newObject =
	(NSManagedObject*)[NSEntityDescription insertNewObjectForEntityForName:class
														  inManagedObjectContext:context];
	
    for (NSString* key in [dict allKeys]) {
        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];
			
            [newObject setValue:relatedObject forKey:key];
        }
        else if ([value isKindOfClass:[NSSet class]]) {
				// This is a to-many relationship
            NSSet* relatedObjectDictionaries = (NSSet*) value;
			
				// Get a proxy set that represents the relationship, and add related objects to it.
				// (Note: this is provided by Core Data)
            NSMutableSet* relatedObjects = [newObject mutableSetValueForKey:key];
			
            for (NSDictionary* relatedObjectDict in relatedObjectDictionaries) {
                NSManagedObject* relatedObject =
				[NSManagedObject createManagedObjectFromDictionary:relatedObjectDict
															   inContext:context];
                [relatedObjects addObject:relatedObject];
            }
        }
        else if (value != nil) {
				// This is an attribute
            [newObject setValue:value forKey:key];
        }
    }
	
    return newObject;
}
@end

SomewhereInYour.m

	
//Set this up as you like. Based on you object model. In the model you have Tables and relationships.
//Pick the ones where you want the recursion to stop, and look for them in the block and return YES
// when you find them.
  BlockingRelationship blockRelationship=^BOOL(id obj,NSString* relationship)){
		NSLog(@"Testing blockRelationship in %@",[obj class]);
		BOOL preventRelationship=NO;
		if ([obj isKindOfClass:NSClassFromString(@"TableA")]){
			if([relationship isEqualToString:@"activeParent"]|[relationship isEqualToString:@"relationshipA"])
				preventRelationship=YES;
		
		}
		else if ([obj isKindOfClass:NSClassFromString(@"TableB")]){ //block all relationships from here.
				preventRelationship=YES;
		
		}
		else if ([obj isKindOfClass:NSClassFromString(@"TableC")]){ //block all relationships from here.
			preventRelationship=YES;
		}
		else if ([obj isKindOfClass:NSClassFromString(@"TableD")]){
			if([relationship isEqualToString:@"aRelationship"]) //many to many - block back 
				preventRelationship=YES;
		}
		else if ([obj isKindOfClass:NSClassFromString(@"TableE")]){
			if([relationship isEqualToString:@"bRelationship"]|([relationship isEqualToString:@"cRelationship"]))
				preventRelationship=YES;
		}
		else if ([obj isKindOfClass:NSClassFromString(@"TableF")]){

				preventRelationship=YES;
		}
		return preventRelationship;
		

	};

	
	
	//this is test code..call to make the dictionary from the managed object.
		NSDictionary *returnedDict=[YourNSObject toDictionaryBlockingRelationships:blockRelationship];

		NSLog(@"%@",returnedDict);
        //create another object model from your NSDictionary.
	YourNSObject *newObjects =(YourNSObject*)[YourNSObject createManagedObjectFromDictionary:returnedDict
																		  inContext:YourNSObject.managedObjectContext];

	
	NSManagedObjectContext *context = templateVersion.managedObjectContext;
	NSError *error = nil;
	if (![context save:&error]) {
		NSLog(@"think we might get here%@",error);
	}	

Just updating about the fix that can be found here: http://stackoverflow.com/questions/6503823/compilation-issue-when-using-blocks-with-llvm-gcc-4-2

Change

BlockingRelationship blockRelationship=^BOOL(id obj,NSString* relationship)){

to

BOOL (^blockRelationship)(id,NSString_)=^(id obj,NSString_ relationship) {

Extremely helpful. Thanks!