Core Data unique attributes

Question

Is it possible to make a Core Data attribute unique, i.e. no two MyEntity objects can have the same myAttribute?

I know how to enforce this programatically, but I'm hoping there's a way to do it using the graphical Data Model editor in xcode.

I'm using the iPhone 3.1.2 SDK.

Answer 1

You could override the setMyAttribute method (using categories) and ensure uniqueness right there, although this may be expensive:

- (void)setMyAttribute:(id)value
{
   NSArray *objects = [self fetchObjectsWithMyValueEqualTo:value];
   if( [objects count] > 0 )  // ... throw some exception
   [self setValue:value forKey:@"myAttribute"];
}

If you want to make sure that every MyEntity instance has a distinct myAttribute value, you can use the objectID of the NSManagedObject objects for that matter.

Answer 2

I've decided to use the validate<key>:error: method to check if there is already a Managed Object with the specific value of <key>. An error is raised if this is the case.

For example:

- (BOOL)validateMyAttribute:(id *)value error:(NSError **)error {
    // Return NO if there is already an object with a myAtribute of value
}

Thanks to Martin Cote for his input.