What's VDOrderedEntityMigrationPolicy?

This is the implementation of a custom Core Data entity migration policy for migrating entities that have ordered relationships (as inherited from Brian Webster's BWOrderedManagedObject).

Ordered relationships in Core Data???

Yes, it can be done. See the excellent work of Brian Webster.

The migration problem with ordered relationships

The order of relationships in BWOrderedManagedObject's implementation is stored in a separate NSArray attribute on the entity class. Since an NSArray has ordering by nature, storing objects as such requires a handle to those objects. The identifier that BWOrderedManagedObject uses in its current implementation is the object's URI representation, which is unique in (and bound to) the managed object context. During migration, objects are replicated in another such context, thereby invalidating any URI references to them.

VDOrderedEntityMigrationPolicy solves that problem by automatically translating those URI's to URI's that are valid in the destination context, as an integral part of the migration process. All of this should be rather transparent for the programmer performing the migration.

Using VDOrderedEntityMigrationPolicy

Since it's a general implementation, you need only specify what ordered relationships you want to migrate. You do this by overriding the abstract implementation of VDOrderedEntityMigrationPolicy and overriding the following method:

- (NSDictionary *)entityMappingForOrderedKeys;

This method should return a dictionary with keys set to the attribute names of all ordered attributes. The values in this dictionary must be the names of the entity mappings that are responsible for copying the child entities for the ordered relationship.

For example, when you want to migrate a model containing Playlist entities that hold an ordered relationship to Track entities, your implementation looks as follows:

PlaylistMigrationPolicy.h: @interface PlaylistMigrationPolicy : VDOrderedEntityMigrationPolicy { } @end

PlaylistMigrationPolicy.m: @implementation PlaylistMigrationPolicy

- (NSDictionary *)entityMappingForOrderedKeys {
	return [NSDictionary dictionaryWithObjectsAndKeys:
	        @"TrackToTrack", @"tracksOrdering",
			nil];
}

@end

That's all. Needless to say, you need to set the custom entity migration policy in your mapping model file to PlaylistMigrationPolicy for the Playlist entity.