Update for Xcode 7 (final):
Prepending the module name to the class (as in Xcode 6 and early beta releases of Xcode 7) is no longer necessary.
The Apple documentation Implementing Core Data Managed Object Subclasses has been
updated accordingly.
The Data Model inspector
has now two fields “Class” and “Module” for an entity:
When you create a Swift managed object subclass for the entity, the
“Module” field is set to “Current Product Module”, and with this setting
creating instances works both in the main application and in unit tests.
The managed object subclass must not be marked with @objc(classname) (this was observed in https://stackoverflow.com/a/31288029/1187415).
Alternatively, you can empty the “Module” field (it will show “None”) and mark the
managed object subclasses with @objc(classname) (this was observed
in https://stackoverflow.com/a/31287260/1187415).
Remark: This answer was originally written for Xcode 6.
There were some changes in the various Xcode 7 beta releases with
respect to this problem. Since it is an accepted answer with many
upvotes and links to it, I have tried to summarize the situation
for the current Xcode 7 final version.
I did both my own “research” and read all the answers to both this question and the similar question
CoreData: warning: Unable to load class named. So attribution goes to all of them, even if I don’t
list them specifically!
Previous answer for Xcode 6:
As documented in Implementing Core Data Managed Object Subclasses, you have to
prefix the entities class name in the Class field in the model entity inspector with the name of your module, for example “MyFirstSwiftApp.User”.