Core Data中的多对一关系的属性声明

问题描述：

我有一个使用Core Data编写的应用程序。我有2个实体与一对多的关系。我有两个子类NSManagedObject。

I have an application written using Core Data. I have 2 entities with a one-to-many relationship. I have subclassed NSManagedObject for both of them. The entity on the one-side is called Playlist and the other is called Song.

播放列表的界面：

@interface VBPlaylist : NSManagedObject {
}

@property (readwrite, copy) NSString *name;

@end

播放列表的实现：

@implementation VBPlaylist

@dynamic name;

@end

我认为我应该有另一个属性来指示播放列表类下的歌曲。我找不到任何示例代码显示一对多关系写作属性。

I think that I should have another property to indicate the Songs under the Playlist class. I cannot find any sample code that shows to-many relationships written as properties. How do you do this?

答

一对一关系建模为Core Data的对象引用。所以从实体 Bar 到实体 Baz （假设 Baz 由 Baz 类实现

To-one relationships are modeled as object references by Core Data. So a to-one relationship from Entity Bar to entity Baz (assuming Baz is implemented by the class Baz) would be

@interface Bar : NSManagedObject {
}

@property (readwrite,nonatomic) Baz * toBaz;

@end

多对多关系被建模为可变集属性（虽然不是 NSMutableSet ）。假设从 Bar 到 Baz 调用 manyBazz ：

To-many relationships are modeled as a mutable set property (though not as an NSMutableSet). Assuming a to-many relationship from Bar to Baz called manyBazz:

@interface Bar : NSManagedObject {

}

@property (retain) NSSet* manyBazz;

@end

// coalesce these into one @interface AnalysisRecord (CoreDataGeneratedAccessors) section
@interface Bar (CoreDataGeneratedAccessors)
- (void)addManyBazzObject:(Baz *)value;
- (void)removeManyBazzObject:(Baz *)value;
- (void)addManyBazz:(NSSet *)value;
- (void)removeManyBazz:(NSSet *)value;

@end

如果要使用NSMutableSet接口来操作 manyBazz 关系，您应该调用 -mutableSetValueForKey：@manyBazz获得符合KVO的代理

If you want to use the NSMutableSet interface to manipulate the manyBazz relationship, you should call -mutableSetValueForKey:@"manyBazz" to get a KVO-compliant proxy for the manyBazz relationship.

在Leopard（OS X 10.5）及更高版本中，所有适当的方法都是在运行时自动生成的Core Data框架，即使你没有显式声明或实现它们（当然，如果你试图使用它们，而不在头文件中声明它们，你会得到一个编译器警告）。因此，你不需要子类

On Leopard (OS X 10.5) and later, all appropriate methods are automaticall generated at run-time by the Core Data framework, even if you do not explicitly declare or implement them (you will, of course, get a compiler warning if you try to use them without declaring them in a header file). Thus you do not need to subclass

获取声明和实现的最简单的方法是在数据建模器中选择属性，并选择将Objective-C 2.0方法声明复制到剪贴板从设计 - >数据模型菜单中粘贴到实现类.h文件中。当然，你必须保持你的.h和模型同步...因此，对于rentzsch的真棒 MO Generator ，一种工具，它将从您的数据模型自动生成（并重新生成）NSManagedObject子类。

The easiest way to get the declaration and implementation right is to select the attributes in the data modeler and choose "Copy Objective-C 2.0 Method Declarations To Clipboard" from the "Design->Data Model" menu, the paste into your implementing classes .h file. Of course, you have to keep your .h and model in sync... hence a hearty recommendation for rentzsch's awesome MO Generator, a tool that will automatically generate (and re-generate) NSManagedObject subclasses from your data model.

Core Data中的多对一关系的属性声明

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