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?
推荐答案
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.
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