I have the model that the images show. In the entity Word I have the attribute name, that I want to be constraint. If I don't make it constraint the model works perfectly, but when I make it constraint it crashes just when opening giving this error. The only code it reads is a DataController in which I just create the container and set mergePolicy to NSmergeByPropertyObjectTrumpMergePolicy.
I think I need name in Word to be constraint. Otherwise when deleting or adding relations with train_list or test_list I get many words with the same name, when I just want one that is added or deleted from these lists.
I don't really know what are the requirements to use a constraint. Maybe there is a conflict with the relationships I have or the delete rules. Any idea?