Saving master data across multiple threads

I'm a bit confused about keeping Core Data multithreading.

I have the following setup NSManagedObjectContext

(same as MagicalRecord

):

SavingContext (NSPrivateQueueConcurrencyType) has child DefaultContext(NSMainQueueConcurrencyType)

      

        
        
        
      

    

Each save thread has its own context ( NSPrivateQueueConcurrencyType

) DefaultContext

as its parent.

So the question is , how can I rely on storing the same type across different threads if I need to guarantee uniqueness?

Here's a small example of a test (Test is a subclass of NSManagedObject):

@implementation Test

+ (instancetype) testWithValue:(NSString *) str {
    [NSThread sleepForTimeInterval:3];
    Test *t = [Test MR_findFirstByAttribute:@"uniqueField" withValue:str];

    if (!t) {
        NSLog(@"No test found!");
        t = [Test MR_createEntity];
    }

    t.uniqueField = str;

    return t;
}
@end

      

        
        
        
      

    

First, it checks to see if Test

the newly created thread context (which has a parent DefaultContext

) exists , and if not, create it in the current thread context.

And here is the test code:

NSOperationQueue *queue = [[NSOperationQueue alloc] init];
queue.maxConcurrentOperationCount = 2;

[queue addOperationWithBlock:^{
    [Test operationWithValue:@"1"];
    [[NSManagedObjectContext MR_contextForCurrentThread] MR_saveToPersistentStoreAndWait];
}];

[queue addOperationWithBlock:^{
    [Test operationWithValue:@"1"];
    [[NSManagedObjectContext MR_contextForCurrentThread] MR_saveToPersistentStoreAndWait];
}];

dispatch_after(dispatch_time(DISPATCH_TIME_NOW, (int64_t)(5 * NSEC_PER_SEC)), dispatch_get_main_queue(), ^{
    NSLog(@"Total tests: %lu", (unsigned long)[Test MR_countOfEntities]);
    [Test MR_truncateAll];
    [[NSManagedObjectContext MR_defaultContext] MR_saveToPersistentStoreAndWait];
});

      

        
        
        
      

    

It just starts two operations and tries to keep the same data. After creating the test, I save all contexts (current thread, context context and root save context). Most of the time there will be 2 tests. You can modify and add a semaphore to ensure that both threads are executing simultaneously.

Update 09/20/2014 I added the code provided by mitrenegade (see below) and now I Test.m

have a function:

-(BOOL)validateUniqueField:(id *)ioValue error:(NSError **)outError {

    // The property being validated must not already exist

    NSFetchRequest *fetchRequest = [NSFetchRequest fetchRequestWithEntityName:NSStringFromClass([self class])];
    fetchRequest.predicate = [NSPredicate predicateWithFormat:@"uniqueField == %@ AND self != %@", *ioValue, self];

    int count = [self.managedObjectContext countForFetchRequest:fetchRequest error:nil];
    if (count > 0) {
        NSLog(@"Thread: %@ (isMain: %hhd), Validation failed!", [NSThread currentThread], [NSThread isMainThread]);
        return NO;
    }

    NSLog(@"Thread: %@ (isMain: %hhd), Validation succeeded!", [NSThread currentThread], [NSThread isMainThread]);
    return YES;
}

      

        
        
        
      

    

With two threads creating the same value (test case is at the beginning of the post) I have the following output:

2014-09-20 11:48:53.824 coreDataTest[892:289814] Thread: <NSThread: 0x15d38940>{number = 3, name = (null)} (isMain: 0), Validation succeeded!
2014-09-20 11:48:53.826 coreDataTest[892:289815] Thread: <NSThread: 0x15e434a0>{number = 2, name = (null)} (isMain: 0), Validation succeeded!
2014-09-20 11:48:53.830 coreDataTest[892:289750] Thread: <NSThread: 0x15e172c0>{number = 1, name = main} (isMain: 1), Validation failed!
2014-09-20 11:48:53.833 coreDataTest[892:289750] Thread: <NSThread: 0x15e172c0>{number = 1, name = main} (isMain: 1), Validation failed!
2014-09-20 11:48:53.837 coreDataTest[892:289750] Thread: <NSThread: 0x15e172c0>{number = 1, name = main} (isMain: 1), Validation failed!
2014-09-20 11:48:53.839 coreDataTest[892:289750] Thread: <NSThread: 0x15e172c0>{number = 1, name = main} (isMain: 1), Validation failed!
2014-09-20 11:48:56.251 coreDataTest[892:289750] Total tests: 2

      

        
        
        
      

    

But if I look at the base sqlite file, then there are no records at all (which means they are stuck in the main context)