相关疑难解决方法(0)

我有一个与单位有"多对多"关系的订单.当我尝试按顺序记录单位(NSSet)时,出现错误错误:

NSFetchRequest *fetchRequest = [[NSFetchRequest alloc] init];
NSEntityDescription *entity = [NSEntityDescription entityForName:@"Order" 
                                          inManagedObjectContext:mainContext];
[fetchRequest setEntity:entity];
NSArray *fetchedObjects = [mainContext executeFetchRequest:fetchRequest 
                                                     error:nil];
for (Order *order in fetchedObjects) {

    NSLog(@"%@", [order units]);
    break;
}        
[fetchRequest release];

结果是:

Relationship 'units' fault on managed object (0x6d9dd00) <Order: 0x6d9dd00> (entity: Order; id: 0x6d88e40 <x-coredata://97A3F3D5-ABA7-499A-A460-5E25CF49C528/Order/p1> ; data: {
    composition = Hemlock;
    condition = "";
    consignee = "";
    consigneeCompanyName = "";
    contactAlternatePhone = "";
    contactEmail = "";
    contactFirstName = "";
    contactLastName = "";
    contactPhone = "";
    customer …