On Jan 3, 2012, at 11:32 PM, Rick Mann wrote: > I'm working on a Core Data-based CAD app. The schema includes Parts and > PartInstances. There is a to-many relationship from Part to PartInstance. The > Library contains Parts, but a CAD file contains Parts and PartInstances. > > When the user creates a new PartInstance in a document, I need to copy the > Part object from the Library to the document. This is not just a copy from > one NSManagedObjectContext to another, it's also a copy from one peristent > store to another. That's fine. > > But when the user instantiates a new PartInstance of the same Part again, > this time I don't want to copy the Part over, because it already exists. I > need a way to determine if the Part object already exists in the document's > MOC. > > I can create a UUID of sorts to use, but I'm wondering if I can do something > to create the new object (in the new persistent store) with the same objectID > as the object from which it's coming. That makes it much easier to then check > for the existence of that object in the new store/context before creating it. > Note that I have several other entities in the overall schema, and it would > be nice not to have to add a uuid to each of those, as well.
There's no way to create an object with the same managedObjectID as an existing object in Core Data, by design: Object IDs are really intended to be unique. An alternate approach might be to have a Part contain two additional attributes: Your own unique ID (such as a UUID), and a "based-on Part" attribute that can contain the original Part's unique ID. So for example, when you put a Part from the library in a document, what really happens is: 1. Create a new Part in the document with the same base attributes as its original Part. 2. Set the new Part's unique ID to a new UUID. (This could be part of #1, e.g. in awakeFromInsert.) 3. Set the new Part's based-on Part to the library Part's unique ID. 4. Create a new PartInstance in the document referring to the Part in the document. This way a Part in a document can be linked back to its original Part in the library, but documents remain self-contained. For performance, you'll probably want to create an index on both the unique ID and the based-on Part ID. If you add a last-modified-date or generation-number to Part, you can use that to detect when you need to propagate changes from the Part in the library to the Part in a document. -- Chris _______________________________________________ Cocoa-dev mailing list (Cocoa-dev@lists.apple.com) Please do not post admin requests or moderator comments to the list. Contact the moderators at cocoa-dev-admins(at)lists.apple.com Help/Unsubscribe/Update your Subscription: http://lists.apple.com/mailman/options/cocoa-dev/archive%40mail-archive.com This email sent to arch...@mail-archive.com
