CoreData 如何管理类关系创建？

发布于 2024-11-16 14:47:06 字数 725 浏览 3 评论 0原文

我有一个实体类，在托管上下文中正确定义。该类与另一个类具有一对多的关系。

Xcode 4 图形工具正确创建了派生类，并且关系由 NSSet 表示。

我想知道如何管理关系类的创建。我的意思是，为了创建主要实体，我使用的是

NSManagedObject *newEntity = [NSEntityDescription
insertNewObjectForEntityForName:@"EntityName"
inManagedObjectContext:context];

但是 NSSet 中的关系怎么样？我是否需要以与父实体相同的方式创建它并定期将其存储在 NSSet 中？

NSManagedObject *child = [NSEntityDescription insertNewObjectForEntityForName:@"ChildName" inManagedObjectContext:context];

NSSet *childSet = [..set creation with child..];
newEntity.child = childSet;

// save newEntity in context

如果是，因为 NSSet 是一个对象，为什么不需要从上下文开始创建它？这样的问题可以应用于实体中的所有“正常”属性，NSString 也是一个对象，为什么一个简单的 newEntity.prop=@"" 就足够了？

原文

I have an entity class, correctly defined in a managed context.
This class has a one-to-many relationship with another class.

Xcode 4 graphic facilities correctly created the derived classes, and the relationship is represented by a NSSet.

I am wondering how the creation of the relation classes is managed.
I mean, for creating the main entity I am using the

NSManagedObject *newEntity = [NSEntityDescription
insertNewObjectForEntityForName:@"EntityName"
inManagedObjectContext:context];

But what about the relationship in NSSet ? Do I need to create it in the same way as a parent entity and store it regularly in NSSet ?

NSManagedObject *child = [NSEntityDescription insertNewObjectForEntityForName:@"ChildName" inManagedObjectContext:context];

NSSet *childSet = [..set creation with child..];
newEntity.child = childSet;

// save newEntity in context

If yes, because NSSet is an object why doesn't it need to be created starting from the context ? Such question could be applied to all the 'normal' properties in the entity, an NSString is an object too, why a simple newEntity.prop=@"" is enough ?

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高冷爸爸 2024-11-23 14:47:06

对于多对多关系，NSManagedObject 有一个 mutableSetValueForKey: 方法，该方法返回您将使用的集合。使用上面定义的 newEntity 和 child 你会做这样的事情：

NSMutableSet *childObjects = [newEntity mutableSetValueForKey:@"childRelationship"];
[childObjects addObject:child];

但是你说你让 Xcode 为你的 Core Data 实体生成自定义子类，所以你有一个方便的方法在 Parent 类中定义，其名称类似于 addChildObject:。使用它，您可以用更简单的版本替换上面的内容，但您还需要将 newEntity 声明为该子类的实例，而不是通用的 NSManagedObject：

Parent *parent = [NSEntityDescription insertNewObjectForEntityForName:@"EntityName" inManagedObjectContext:context];
NSManagedObject *child = [NSEntityDescription insertNewObjectForEntityForName:@"ChildName" inManagedObjectContext:context];

[parent addChildObject:child];

For to-many relationships NSManagedObject has a mutableSetValueForKey: method that returns the set you would use. With newEntity and child defined as above you'd do something like this:

NSMutableSet *childObjects = [newEntity mutableSetValueForKey:@"childRelationship"];
[childObjects addObject:child];

But you said you had Xcode generate custom subclasses for your Core Data entities, so you have a convenience method defined in the Parent class which would be named something like addChildObject:. Using that you could replace the above with a simpler version, but you'll also need to declare newEntity as an instance of this subclass instead of as a generic NSManagedObject:

Parent *parent = [NSEntityDescription insertNewObjectForEntityForName:@"EntityName" inManagedObjectContext:context];
NSManagedObject *child = [NSEntityDescription insertNewObjectForEntityForName:@"ChildName" inManagedObjectContext:context];

[parent addChildObject:child];

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