CoreDataでatomicにsaveする ref: http://qiita.com/kenmaz/items/89e50f94262a4d0547c5

file0.txt

User
-uid
-name

file1.txt

[
  { 
    "uid": 1, 
    "name": "kentaro"
  },
  {
     "uid": 2,
     "name": "yamada"
  }
  ...
]

file2.m

//APIアクセスしてレスポンス取得
NSNumber* uid = ..;
NSString* name = ...

[MagicalRecord saveWithBlock:^(NSManagedObjectContext *ctx) {
    User* user = [User MR_findFirstByAttribute:@"uid" withValue:uid inContext:ctx];
    if (user == nil) {
        user = [User MR_createInContext:ctx];
        user.uid = uid;
    }
    person.name = name;
} completion:^(BOOL success, NSError *error) {
    assert(success);
}];

file3.m

int cnt = [Person MR_countOfEntitiesWithPredicate:[NSPredicate predicateWithFormat:@"uid = 1"]];
NSLog(@"count = %d", cnt);

//> count:100

file4.m

static dispatch_queue_t serialQueue;
static dispatch_once_t onceToken;
dispatch_once(&onceToken, ^{
    serialQueue = dispatch_queue_create("net.kenmaz.serialqueue", DISPATCH_QUEUE_SERIAL);
});

dispatch_async(serialQueue, ^{
    //ここから〜
    NSManagedObjectContext* ctx = [NSManagedObjectContext MR_context];

    User* user = [User MR_findFirstByAttribute:@"uid" withValue:uid inContext:ctx];
    if (user == nil) {
        user = [User MR_createInContext:ctx];
        user.uid = uid;
    }
    person.name = name;

    [ctx MR_saveToPersistentStoreAndWait];
    //〜ここまで排他的に実行
});

file5.m

int cnt = [Person MR_countOfEntitiesWithPredicate:[NSPredicate predicateWithFormat:@"uid = 1"]];
NSLog(@"count = %d", cnt);

//> count:1