I just checked with the latest version of Xcode (Beta 5) and this issue is still present.
diegooo
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Just posted a Feedback for this (FB12759179)
Yes, @kharrison I noticed that when trying this in beta 5. I did almost the same as you, just copied the name of the migration inside the string. This is just a blind guess because I see absolutely no docs about this. I then tried a couple of different initializers with no success BUT... I think I got lightweight migrations working.
Amazingly, they seem to work if you just modify the @Model class directly and re-run the app. You don't have to specify a migration or anything. Just provide default values for new attributes and it works!!!
If you don't provide default values, the app will fail to start with an error that the migration failed.