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.
When querying, make sure you use
self.managedObjectContext, to ensure you check for duplicate's existence in correctmanagedObjectContext. Otherwise, Save As (that is, migration to new url) will fail.Performing a fetch will change the object graph in the managed object context - which is not something you want to do inside a validation method!
not necessarily - if you only query but not insert/update/delete any entity, the context will not get dirty by the validation. It is not recommended to introduce edits (to dirty your context) in validations, because it causes recursion - still in some situations it is OK.
You could optimize by checking if previously saved and field not changed.