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.
The interface for Playlist:
@interface VBPlaylist : NSManagedObject {
}
@property (readwrite, copy) NSString *name;
@end
The implementation for Playlist:
@implementation VBPlaylist
@dynamic name;
@end
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?
Best Answer
To-one relationships are modeled as object references by Core Data. So a to-one relationship from Entity
Bar
to entityBaz
(assumingBaz
is implemented by the classBaz
) would beTo-many relationships are modeled as a mutable set property (though not as an
NSMutableSet
). Assuming a to-many relationship fromBar
toBaz
calledmanyBazz
: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 themanyBazz
relationship.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
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.