BrokenVersion.m

    [MagicalRecord saveWithBlock:^(NSManagedObjectContext* localContext) {
      LSUser* currentUser = nil;
      NSDictionary* apiKeyDic = [[ACSimpleKeychain defaultKeychain] credentialsForIdentifier:@"api_key" service:@"MerchantCenter"];
      if( apiKeyDic != nil ) {
        NSString* email = [apiKeyDic objectForKey:ACKeychainUsername];
        if( email != nil ) {
          currentUser = [LSUser MR_findFirstByAttribute:LSUserAttributes.email withValue:email inContext:localContext];
          NSLog(@"USER HAS VOUCHERS: %ld", [LSVoucher MR_countOfEntitiesWithPredicate:[NSPredicate predicateWithFormat:@"user = %@", currentUser]]);
        }
      }
      
      NSArray* items = [responseObject objectForKey:@"vouchers"];
      NSArray* vouchers = [LSVoucher MR_importFromArray:items inContext:localContext];
      for(LSVoucher* v in vouchers) {
        if( [[v.user objectID] isEqual:[currentUser objectID]] == NO)
          v.user = currentUser;
      }
    }];

FixedVersion.m

NSManagedObjectContext* ctx = [NSManagedObjectContext MR_contextThatPushesChangesToDefaultContext];
 LSUser* currentUser = nil;
 NSDictionary* apiKeyDic = [[ACSimpleKeychain defaultKeychain] credentialsForIdentifier:@"api_key" service:@"MerchantCenter"];
 if( apiKeyDic != nil ) {
   NSString* email = [apiKeyDic objectForKey:ACKeychainUsername];
   if( email != nil ) {
     currentUser = [LSUser MR_findFirstByAttribute:LSUserAttributes.email withValue:email inContext:ctx];
     NSLog(@"USER HAS VOUCHERS: %ld", [LSVoucher MR_countOfEntitiesWithPredicate:[NSPredicate predicateWithFormat:@"user = %@", currentUser]]);
   }
 }
 
 NSArray* items = [responseObject objectForKey:@"vouchers"];
 for(NSDictionary* item in items) {
   LSVoucher* v = [LSVoucher MR_importFromObject:item inContext:ctx];
   if( [[v.user objectID] isEqual:[currentUser objectID]] == NO)
     v.user = currentUser;
   
   [ctx MR_save];
   NSLog(@".");
 }
 [ctx MR_saveNestedContexts];