***解决方案：获取子实体在获取的结果控制器中，按 parent 过滤。

Best solution: Fetch the Child entity in your fetched results controller, filter by parent.

对于插入，使用 NSFetchedResultsControllerDelegate 的纯香草实现。这将提供一个很好的动画或让你添加自己的。

For inserts, use the plain vanilla implementation of the NSFetchedResultsControllerDelegate. This will provide a nice animation or let you add your own.