在 Core Data 中设置父子关系 [英] Setting up a parent-child relationship in Core Data

问题描述

我正在尝试在 Core Data 中建立关系.我有一个树列表，每棵树都有一个水果列表.所以我有一个 Tree 实体和一个 Fruit 实体.

I'm trying to set up a relationship in Core Data. I have a list of Trees, and each Tree will have a list of Fruits. So I have a Tree entity and a Fruit entity.

在代码中，我想在表格视图中列出树.当您单击一棵树时，它应该会显示该树上生长的水果列表.

In code, I will want to list the Trees, in a table view for example. When you click on a Tree, it should then display a list of fruits that growing on that Tree.

我如何建立这种关系?我需要给 Fruit 一个名为 tree 的属性吗?以及如何在代码中设置关系，例如，当我创建一个 Fruit 时，我如何将它与给定的 Tree 相关联?

How do I set up this relationship? Do I need to give the Fruit an attribute called tree? And how do I set the relationship in code, for example when I create a Fruit how do I associate it with a given Tree?

推荐答案

Soleil,

这很简单.首先，您的模型应如下所示(为了简单起见，我跳过了属性).

This is quite simple. First of of all, your model should look like the following (for the sake of simplicity I skipped attributes).

在这种情况下，Tree 可以有零个或多个 Fruit(参见 fruits 关系).相反，一个 Fruit 有一个 tree 关系(逆关系).

In this case a Tree can have zero or more Fruits (see fruits relationship). On the contrary, a Fruit has a tree relationship (an inverse relationship).

特别是，fruits 关系应该如下所示

In particular, the fruits relationship should look like the following

在这里，您可以看到已设置一对多关系.删除规则意味着如果你移除一棵树，它的果实也会被删除.

Here, you can see that a to-many relationship has been set. The Delete rule means that if you remove a tree, also its fruits will be deleted.

tree 关系如下

这是一对一的关系，因为水果只有附着在树上才能存在.可选 标志未设置.所以，当你创建一个水果时，你还需要指定它的父级(在这种情况下是一棵树).Nullify 规则意味着当您删除一个水果时，Core Data 不会删除与该水果关联的树.它只会删除您指定的水果.

This is a one-to-one relationship since a fruit can exist only if attached to a tree. Optional flag is not set. So, when you create a fruit you also need to specify its parent (a tree in this case). Nullify rule means that when you delete a fruit, Core Data will not delete the tree associated with that fruit. It will delete only the fruit you specified.

当你创建一个 Fruit 实体时，你应该遵循类似的路径

When you create a Fruit entity you should follow a similar path

NSManagedObject *specificFruit = [NSEntityDescription insertNewObjectForEntityForName:@"Fruit" inManagedObjectContext:context];
[specificFruit setValue:parentTree forKey:@"tree"];

或者如果您创建了 NSManagedObject 子类:

or if you have create NSManagedObject subclasses:

Fruit *specificFruit = [NSEntityDescription insertNewObjectForEntityForName:@"Fruit" inManagedObjectContext:context];
specificFruit.tree = parentTree;

希望有所帮助.

附言检查代码，因为我是在没有 Xcode 支持的情况下编写的.

P.S. Check the code since I've written without Xcode support.

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