CoreData - 一对多的建模关系是一对一的 [英] CoreData - one-to-many modeled relationship comes out as one-to-one

问题描述

我是 Core Data 建模的新手，我很难理解一对多关系的工作原理.

I am new to Core Data modeling, and I am having a hard time understanding how one-to-many relationships work.

我有一个名为 Task 的父实体，它可以有多个 Comment 实体的实例.我这样建模:在Comments 上，与Task 的关系称为task，Task 实体是一个目的地.在Task上，有一个叫做comments的关系，以Comment为目的地，两个关系互为逆.

I have a parent entity called Task, which can have several instances of Comment entity. I modeled it like this: on Comments, a relationship to Task called task with the Task entity a destination. On Task, a relationship called comments, with Comment as its destination, and both relationships are each others inverse.

不定义反向会导致警告或错误消息.在以这种方式建模时，我注意到一旦我为给定的 Task 创建了第二条评论，第一个评论就会被替换(一对一关系).

Not defining an inverse results in either warnings or error messages. While modeling this way works, I've noticed that once I create a second comment for a given Task, the first is replaced (one-to-one relationship).

告诉核心数据模型这种关系允许在一个Task中有很多评论的正确方法是什么?

What would be the correct way to tell the Core Data Model that this relationship allows many comments in one Task?

此外，由于 CoreData 似乎自己管理主键，我将如何创建 NSPredicate 来检索给定任务的所有评论?

Also, since CoreData seems to manage primary keys on its own, how would I create an NSPredicate to retrieve all comments for a given Task?

感谢您的任何建议！

推荐答案

首先你需要在Task实体中设置plural选项，在你的.xdatamodeled 并选择Task 实体的relationship 属性到comments 你应该可以看到这个

First of all you need to set the plural option in the Task entity, select it in your .xdatamodeled and select the relationship property of Task entity to comments you should be able to see this

有一个 plural 选项，请务必检查一下.您必须重新创建 NSManagedObject 如果您使用生成的类，并且如果您使用 sqlite 存储，您必须删除和重建，这样它就不会抱怨新架构与旧架构不同.

there is a plural option be sure to check that out. You must recreate your NSManagedObject if your using generated classes and also if your using sqlite store you must delete and rebuild so it will not complain about the new schema not being the same with the old one.

要检查您是否有一对多关系，您的 Task 实体应该有一个名为 comments 的属性，它是 NSSet 的类类型，而不是 Comments.

To check if you have a one to many relationship your Task entity should have a property called comments which is a class type of NSSet not Comments.

如果您想检索给定任务的所有评论，您需要迭代该任务的 NSSet(comments) 属性.

If you want to retrieve all comments for a given task you need to iterate the NSSet(comments) property of that task.

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