如何在 Swift 中定义 CoreData 关系? [英] How to define CoreData relationship in Swift?

问题描述

在 CoreData 中，我定义了一个从 Node 到 Tag 的无序对多关系.我已经创建了一个这样的 Swift 实体:

In CoreData, I have defined an unordered to-many relationship from Node to Tag. I've created an Swift entity like this:

import CoreData
class Node : NSManagedObject {
    @NSManaged var tags : Array<Tag>
}

现在我想给 Node 的一个实例添加一个 Tag，像这样:

Now I want to add a Tag to an instance of Node, like this:

var node = NSEntityDescription.insertNewObjectForEntityForName("Node", inManagedObjectContext: managedObjectContext) as Node
node.tags.append(tag)

但是，这失败并出现以下错误:

However, this fails with the following error:

由于未捕获的异常NSInvalidArgumentException"而终止应用程序，原因:对多关系的值类型不可接受:属性 =标签"；所需类型 = NSSet;给定类型 = _TtCSs22ContiguousArrayStorage000000000B3440D4;价值 = (<_TtC8MotorNav3Tag:0xb3437b0>(实体:标签；id:0xb343800；数据:{...})").'

Terminating app due to uncaught exception 'NSInvalidArgumentException', reason: 'Unacceptable type of value for to-many relationship: property = "tags"; desired type = NSSet; given type = _TtCSs22ContiguousArrayStorage000000000B3440D4; value = ( "<_TtC8MotorNav3Tag: 0xb3437b0> (entity: Tag; id: 0xb343800 ; data: {...})" ).'

多对多关系的正确类型是什么?

What is the correct type for to-many relationships?

推荐答案

为了能够在 Swift 中处理一对多关系，您需要将属性定义为:

To be able to work with one-to-many relationship in Swift you need to define property as:

class Node: NSManagedObject {
    @NSManaged var tags: NSSet
}

如果您尝试使用 NSMutableSet 更改将不会保存在 CoreData 中.当然推荐在Node中定义反向链接:

If you try to use NSMutableSet changes will not be saved in CoreData. And of course it is recommended to define reverse link in Node:

class Tag: NSManagedObject {
    @NSManaged var node: Node
}

但是 Swift 仍然无法在运行时生成动态访问器，因此我们需要手动定义它们.在extension 类中定义它们并放入Entity+CoreData.swift 文件中非常方便.下面是Node+CoreData.swift文件的内容:

But still Swift cannot generate dynamic accessors in runtime, so we need to define them manually. It is very convenient to define them in class extension and put in Entity+CoreData.swift file. Bellow is content of Node+CoreData.swift file:

extension Node {
    func addTagObject(value:Tag) {
        var items = self.mutableSetValueForKey("tags");
        items.addObject(value)
    }

    func removeTagObject(value:Tag) {
        var items = self.mutableSetValueForKey("tags");
        items.removeObject(value)
    }
}

用法:

// somewhere before created/fetched node and tag entities
node.addTagObject(tag)

重要提示:要使这一切正常工作，您应该验证 CoreData 模型中实体的类名称是否包含您的模块名称.例如.MyProjectName.Node

Important: To make it all work you should verify that class names of entities in you CoreData model includes your module name. E.g. MyProjectName.Node

这篇关于如何在 Swift 中定义 CoreData 关系?的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！