Question

I was under the impression that with key-value coding, I could set any undefined attribute on a NSManagedObject at runtime and no exception would be thrown, but it wouldbe a way to hold objects attached to the model that are not in the data model.

For example, I have a Foo object that does not have a “bar” attribute. I though that at runtime, I could set it like so:

Foo *foo = [NSEntityDescription insertNewObjectForEntityForName:@"Foo" inManagedObjectContext:ManagedObjectContext];
[foo setValue:@"foobar" forUndefinedKey:@"bar"];

I would then expect the ManagedObjectContext to hold this value until I queried it later, but a save on the Managed Object Context would not store the bar value.

The famous error keeps popping up when I run this code:

the entity Foo is not key value coding-compliant for the key "bar"

..so my question is: What am I not getting or doing wrong?

Answer 1

This question, NSManagedObject subclasses and setValuesForKeysWithDictionary:, might be relevant.

Conceptually, I don’t think you can assign random values and keys safely. Doing so would seem likely to disrupt the object graph.