Thread-safe NSManagedObjectContext factory.

AZModel.h

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

@interface AZModel : NSObject
+ (AZModel *)sharedModel;
- (__strong NSManagedObjectContext *)managedObjectContextForCurrentThread;
- (__autoreleasing NSManagedObjectContext *)newManagedObjectContextForCurrentThread;
@end

AZModel.m

#import "AZModel.h"
#import <objc/runtime.h>

@implementation AZModel

#define kAZModelModelName @"DataModel"

+ (AZModel *)sharedModel
{
	static AZModel *instance;
	static dispatch_once_t onceToken;
	dispatch_once(&onceToken, ^{
		instance = [[AZModel alloc] init];
	});
	return instance;
}

#pragma mark - Managed object context

- (__strong NSManagedObjectContext *)managedObjectContextForCurrentThread
{	
	static char key;
	// That is the object we will use to associate managedObjectContext with
	NSThread *currentThread = [NSThread currentThread];

	// If there was one created before, return it
	NSManagedObjectContext *moc = objc_getAssociatedObject(currentThread, &key);
	if (moc) {
		return moc;
	} 

	// Otherwise create a new one and configure it
	moc = [[NSManagedObjectContext alloc] init];
	moc.persistentStoreCoordinator = [self persistentStoreCoordinator];
	// Configure your moc here, for example:
	moc.undoManager = nil;
	moc.mergePolicy = NSMergeByPropertyObjectTrumpMergePolicy;

	// Associate it with current thread, so it will be deallocated as soon as thread deallocated
	objc_setAssociatedObject(currentThread, &key, moc, OBJC_ASSOCIATION_RETAIN);	
	return moc;
}

- (__autoreleasing NSManagedObjectContext *)newManagedObjectContextForCurrentThread
{
	__autoreleasing NSManagedObjectContext *moc = [[NSManagedObjectContext alloc] init];
	moc.persistentStoreCoordinator = [self persistentStoreCoordinator];
	moc.undoManager = nil;
	moc.mergePolicy = NSMergeByPropertyObjectTrumpMergePolicy;	
	return moc;
}

- (NSPersistentStoreCoordinator *)persistentStoreCoordinator
{
	static NSPersistentStoreCoordinator *PersistentStoreCoordinator;
	static dispatch_once_t onceToken;
	dispatch_once(&onceToken, ^{
		NSURL *url = [[NSBundle mainBundle] URLForResource:kAZModelModelName withExtension:@"momd"];
		
		NSManagedObjectModel *mom = [[NSManagedObjectModel alloc] initWithContentsOfURL:url];
		
		NSString *storeURLString = [NSSearchPathForDirectoriesInDomains(NSDocumentDirectory, NSUserDomainMask, YES) lastObject];
		storeURLString = [storeURLString stringByAppendingPathComponent:kAZModelModelName];
		storeURLString = [storeURLString stringByAppendingPathExtension:@"sqlite"];
		
		NSURL *storeURL = [NSURL fileURLWithPath:storeURLString];
		
		NSError *error = nil;
		PersistentStoreCoordinator = [[NSPersistentStoreCoordinator alloc] initWithManagedObjectModel:mom];
		mom = nil;
		
		NSDictionary *options = [[NSDictionary alloc] initWithObjectsAndKeys:
								 [NSNumber numberWithBool:YES], NSMigratePersistentStoresAutomaticallyOption,
								 [NSNumber numberWithBool:YES], NSInferMappingModelAutomaticallyOption, nil];
		
		NSPersistentStore *persistentStore = [PersistentStoreCoordinator addPersistentStoreWithType:NSSQLiteStoreType 
																					  configuration:nil 
																								URL:storeURL 
																							options:options 
																							  error:&error];
		if (!persistentStore) {
			persistentStore = [PersistentStoreCoordinator addPersistentStoreWithType:NSSQLiteStoreType 
																	   configuration:nil 
																				 URL:storeURL 
																			 options:options 
																			   error:&error];		
		}
		ZAssert(persistentStore != nil, @"Error initializing PSC: %@\n%@", [error localizedDescription], [error userInfo]);
	});
	return PersistentStoreCoordinator;
}

@end