CoreData - ChildContext save push ParentContext

Question

问题：

NSManagedObjectContext* mainContext = [[NSManagedObjectContext alloc] initWithConcurrencyType:NSMainQueueConcurrencyType];

[context setPersistentStoreCoordinator:coordinator];

NSManagedObjectContext* childContext = [[NSManagedObjectContext alloc] initWithConcurrencyType:NSPrivateQueueConcurrencyType];

[childContext setParentContext:mainContext];

Amount* amount = [[Amount alloc] initWithEntity:[NSEntityDescription entityForName:@"Amount" inManagedObjectContext:context] insertIntoManagedObjectContext:nil];

amount.xyz = @"String";
[childContext insertObject:amount];

/*
(lldb) po [childContext insertedObjects]
{(
    <Amount: 0x7fc3635b90f0> (entity: Amount; id: 0x7fc3635b7a00 <x-coredata:///Amount/t3176AD5B-914D-469D-AFCE-21D5D679E3652> ; data: {
    xyz = String;
})
)}
*/
NSError* error = nil;
[childContext save:&error];

childContext MO被推送到mainContext，但是为什么xyz是nil并且它不是＆＃34; String＆＃34;？我很苦恼！

/*
(lldb) po [mainContext insertedObjects]
{(
    <Amount: 0x7fc3634701e0> (entity: Amount; id: 0x7fc3635b7a00 <x-coredata:///Amount/t3176AD5B-914D-469D-AFCE-21D5D679E3652> ; data: {
    xyz = nil;
})
)}
*/