在Core Data中设置父子关系

问题描述：

我想在Core Data中建立关系。我有一个树列表，每个树将有一个水果列表。所以我有一个树实体和水果实体。

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，我将列出树，例如在表视图中。当你点击一个树，它应该显示生长在树上的水果列表。

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.

如何设置这种关系？我需要给 Fruit 一个称为tree的属性吗？如何在代码中设置关系，例如当我创建一个水果如何将它与给定的树

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?

答

Soleil，

首先，你的模型应该看起来像下面（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，树可以有零个或多个水果（参见 fruits 关系）。相反， Fruit 具有树关系（反向关系）。

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).

特别地， fruits 关系应如下所示：

In particular, the fruits relationship should look like the following

在这里，您可以看到，许多关系已经设定。

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.

树

这是一个一对一的关系，因为水果只有连接到一棵树才能存在。未设置可选标志。所以，当你创建一个水果，你还需要指定其父（在这种情况下一棵树）。 Nullify 规则意味着当您删除水果时，Core Data不会删除与该水果相关联的树。

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.

当您创建一个水果实体时，您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"];

或者如果您有create NSManagedObject

or if you have create NSManagedObject subclasses:

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

希望有帮助。

检查代码，因为我写的没有Xcode支持。

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

在Core Data中设置父子关系

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