CoreData unique constraints disappears in Xcode

Question

What I want to do:

Add a unique constraint on the attribute photoID in the entity PhotoUpload.

Problem:

I tap on constraints a

Answer 1

I believe this to be a bug in Xcode. If you change the model to add (or amend or remove) a constraint, the underlying model file is not updated, even though the screen shows the changes correctly. To overcome it, change another element of the model at the same time as adding the uniqueness constraint. Save the model. Then change the other element back and save again.