CoreData如何管理类关系创建？ [英] how does CoreData manage class relationship creation?

问题描述

我有一个实体类，在托管上下文中正确定义。
这个类与另一个类有一对多的关系。

Xcode 4图形工具正确地创建了派生类，关系用一个NSSet。

我想知道如何管理关系类的创建。
我的意思是，为创建主实体我使用

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

但是NSSet中的关系呢？我需要以与父实体相同的方式创建它并将其定期存储在NSSet中吗？

  NSManagedObject * child = [NSEntityDescription insertNewObjectForEntityForName：@ChildNameinManagedObjectContext：context]; 
 
 NSSet * childSet = [..set creation with child ..]; 
 newEntity.child = childSet; 
 
 //在上下文中保存newEntity 
  

如果是，对象为什么不需要从上下文开始创建？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 = @就够了？

解决方案

对于多对多关系NSManagedObject有一个 mutableSetValueForKey：方法返回您将使用的集合。使用上面定义的 newEntity 和 child ，您可以这样做：

  NSMutableSet * childObjects = [newEntity mutableSetValueForKey：@childRelationship]; 
 [childObjects addObject：child];但是你说你有Xcode为你的核心数据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，类将被命名为 addChildObject：。使用它，你可以用一个更简单的版本替换上面，但你还需要声明 newEntity 作为这个子类的实例，而不是一个通用的NSManagedObject：
  Parent * parent = [NSEntityDescription insertNewObjectForEntityForName：@EntityNameinManagedObjectContext：context]; 
 NSManagedObject * child = [NSEntityDescription insertNewObjectForEntityForName：@ChildNameinManagedObjectContext：context]; 
 
 [parent addChildObject：child]; 
  
 
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 ? 
解决方案
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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