关于 访问一对一关系引发了崩溃 的问题

有两个表Assets和Rewards，对应关系是一对一。我想访问reward表。
NSFetchRequest *request = [NSFetchRequest fetchRequestWithEntityName:@"Assets"];
NSPredicate *predicate = [NSPredicate predicateWithFormat:@"assetsId == %@",str];
[request setPredicate:predicate];
[self.managedObjectContext executeFetchRequest:request onSuccess:^(NSArray *assetsArr) {
for (Assets *assets in assetsArr) {
NSLog(@"assets %@",[assets valueForKey:@"assetrewards.rewardsId"]);
}
} onFailure:^(NSError *error) {

    NSLog(@"Error fetching: %@", error);

}];

崩溃报告：

:  Terminating app due to uncaught exception 'NSUnknownKeyException', reason: '[<Assets 0x9fb9380> valueForUndefinedKey:]: the entity Assets is not key value coding-compliant for the key "assetrewards.rewardsId".'

检测：
`NSLog(@"assets %@",[assets valueForKey:@"assetrewards"]);
`

得到的输出结果：

assets <Rewards: 0xad8a4f0> (entity: Rewards; id: 0x9fe4bf0 <x-coredata://B2D33481-2AF2-4C6D-9428-33BDCF653881-15578-00004A2B32ACC29E/Rewards/pc05848de529d4cbab6eb4475a5f9f591> ; data: <fault>)

我想要通过对应关系访问reward内的所有值，或者是一个特定的asset可以得到对应reward的详细内容。