How to add unique constraints for some fields in Core Data

Question

I use Xcode for iOS development. I have some entity (for example, User), and I need to set unique constraint for his name, but I can't find how I can do it through visual editor. Is it possible to do it through GUI? Or it's possible through code only? I will be glad to get some screenshot.

Answer 1

There's a new section in the sidebar when selecting an entity in the editor for Core Data. You can set what constraint(s) you want to be unique across all instances of an entity

For automatic conflict resolution during saves, you'll need to make sure you've got a merge policy set for your managed object context or else you'll just get errors when saving (which might actually be what you want)

[managedObjectContext setMergePolicy:NSMergeByPropertyObjectTrumpMergePolicy]; 

The "Swift version" is exactly the same

managedObjectContext.mergePolicy = .mergeByPropertyObjectTrumpMergePolicyType 

Keep in mind conflict resolution only happens during saves, and not inserts. So if you're making use of a NSFetchedResultsController you will see entities with non-unique constraints as they're inserted.

If you want to sure you have no entities with non-unique constraints in your managed object context without saving (if you're making use of a FRC), this answer is still probably the best way to go. Although, keep in mind, it's expensive if you're doing a lot of inserts, since NSFetchRequests are expensive operations.

Sample code for this demo can be found here

Answer 2

Swift solution:

As noted in the other answer, you can have unique constraints in Core Data for iOS9 onwards.

To do this, first add constraints to the Entity from the Core Data Editor (explaination in Zachary's answer).

Then add this line in code:

managedObjectContext.mergePolicy = NSMergeByPropertyObjectTrumpMergePolicy 

Note: The uniqueness is checked when you do managedObjectContext.save() not while you're just adding objects to the managed object.

NSMergeByPropertyObjectTrumpMergePolicy is just one of the merge policies, which overwrites the old value with the new one. You might want to check the documentation for other options.