Question

I have a Core Data database containing (among others) WorkingPlan and Position. A WorkingPlan has zero or more Position objects.
Everything works fine, until I am trying to add the same Position to one WorkingPlan multiple times. Instead of having multiple relations (as I would need it to be), i only get one relation. What can I do to get multiple relations to that Position?

Answer 1

As already mentioned in the comments, a Core Data relationship describes the relations of one object to a set of distinct other objects. It is not possible for one object to have more than one relationship to the same other object.

One possible solution that comes into my mind is to introduce another entity WorkingStep between WorkingPlan and Position:

• steps is a ordered to-many relationship from WorkingPlan to WorkingStep,
• plan is the inverse to-one relationship,
• position is a to-one relationship from WorkingStep to Position,
• steps is the inverse to-many relationship.

I have suggested an ordered relationship from WorkingPlan to WorkingStep, because I assume that the steps for one plan must be executed in a defined order.

For example, if the positions “pos1”, “pos2”, “pos1” have to be done for a plan in that order, you would add 3 steps “step1”, “step2”, “step3” to the plan, and both “step1” and “step3” are related to “pos1”, and “step2” is related to “pos2”.

Adding values to an ordered relationship is a bit tricky. The following code shows how to create the objects described above:

WorkingPlan *plan1 = [NSEntityDescription insertNewObjectForEntityForName:@"WorkingPlan" inManagedObjectContext:context];

WorkingStep *step1 = [NSEntityDescription insertNewObjectForEntityForName:@"WorkingStep" inManagedObjectContext:context];
WorkingStep *step2 = [NSEntityDescription insertNewObjectForEntityForName:@"WorkingStep" inManagedObjectContext:context];
WorkingStep *step3 = [NSEntityDescription insertNewObjectForEntityForName:@"WorkingStep" inManagedObjectContext:context];

Position *pos1 = [NSEntityDescription insertNewObjectForEntityForName:@"Position" inManagedObjectContext:context];
Position *pos2 = [NSEntityDescription insertNewObjectForEntityForName:@"Position" inManagedObjectContext:context];

step1.position = pos1;
step2.position = pos2;
step3.position = pos1;

// temporary proxy object used to modify the ordered to-many relationship "steps":
NSMutableOrderedSet *tmpMutableSteps = [plan1 mutableOrderedSetValueForKey:@"steps"];
[tmpMutableSteps addObject:step1];
[tmpMutableSteps addObject:step2];
[tmpMutableSteps addObject:step3];

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More information:

You can also set plan1.steps in one “step” like this:

plan1.steps = [NSOrderedSet orderedSetWithObjects:step1, step2, step3, nil]; // works!

But some accessor methods do not work with ordered relationships:

[plan1 addStepsObject:step1]; // does not work!

seems to be the “logical step”, but it throws an NSInvalidArgumentException:

*** -[NSSet intersectsSet:]: set argument is not an NSSet

This seems to be a bug in the auto-generated accessor methods which has already been noticed by other people (e.g. Exception thrown in NSOrderedSet generated accessors). Using the proxy object is a workaround for that problem.