Core Data Model

Question

I'm struggling with creating a suitable Core Data model for my app. I'm hoping someone here can provide some guidance.

I have two entities -- "Goals" and "Items". The Goals entity contains only a goal description, but any goal may have any number of subgoals, and these may extend multiple levels in a tree structure. Subgoals are to be contained within the same entity, so presumably the Goal entity will contain a pointer to "parent" which will be the parent goal of any subgoal.

There will also be an "Items" entity that contains a couple of text fields and a couple of binary items, and must be linked (ideally, by a unique identifier, perhaps objectID) to the particular goal or subgoal the item(s) are related to.

I am totally fumbling with how to set this model up. I know what attributes need to be in each entity, but the relationships, particularly between goals and "subgoals", has me stumped. I don't seem to be able to turn up any good examples of tree structures in Core Data on the Internet, and even the couple of books I have on Core Data don't seem to address it.

Can anyone here help an old SQL programmer get headed the right direction with these relationships in Core Data? Thanks.

Answer 1

Have you tried creating a one-to-many from Goal to itself, and a one-to-one from Goal to Item? The only thing I would worry about here is circular references.

Also, read Relationships and Fetched Properties in the CoreData Programming Guide.

Answer 2

Here is how it is done:

You set up a to-many relationship from Goal to Item in the model editor. Don't use any ids, foreign keys etc. This is old-fashioned database thinking - you can forget about it. Here we are only dealing with an object graph. The database layer is just an implementation detail for persisting the data.

Make two more relationships in entity Goal to itself: a to-one called parent, a to-many called subGoals. Make them the inverse of each other. Simple!

Answer 3

QED is correct, you can create a to many relationship on goal (call it subgoals) as well as a to-one relationship on goal (call it parentGoal) and set them as inverses to each other.

Then create another to many relationship (call it items) on the goal entity, with the inverse being a to one relationship on the item entity (call it goal). Then you're all set. You don't need to link items with a unique id, just add them to the items relationship.

Also note that if you did want to give items a unique id, do not use the objectID. The objectID should only be used as a temporary id as they are not guaranteed to remain the same. In fact they will change if you ever do a Core Data migration.

Answer 4

One way, though not really great, is to create a another entity, say subGoal, and each goal has one subGoal and each object of subGoal has many goal.