TL;DR
At least as of Xcode 8/9, open the mapping model then from the Editor menu select Refresh data models. Usually it seems you need to restart Xcode. If that doesn't do it you might try re-selecting the destination at the bottom of the model editor.
More Tips
Definitely NEVER change a model after it has been distributed in an app build.
For this example, let's say you have published Data Model 1 (DM1) and are making a migration to DM2. If you set DM2 as the active version then run your app, a migration will run on your persistent store. If you then make another change to DM2, run your app... Boom!
The issue is that your store has already been migrated to "DM2" but the data in the store doesn't fit into the model anymore. And, we can't migrate from DM2 to DM2 again.
It may seem like an obvious solution to go ahead and create DM3. It is
usually a good idea though to minimize the number of models and
migrations while you are developing.
So... now you have a persistent store that has been migrated to a defunct DM2. How do you test the migration again? You could revert your app and generate some data with DM1 but I prefer to use backups
Creating a backup
Before you run your app with DM2 you can copy the existing store (with DM1) to use for later test migrations. On macOS you can easily do this manually. The code below should do the trick as well. Typically you wouldn't want to ship this, rather you could just put it somewhere before your normal CD stack opens, run the app, then stop the app (maybe place a breakpoint just after then end the run via Xcode).
let fm = FileManager.default
let url = // The store URL you would use in ↓
// try coordinator.addPersistentStore(ofType: NSSQLiteStoreType, configurationName: nil, at: url, options: nil)
let dir = url.deleteLastPathComponent().appendingPathComponent("Backup", isDirectory: true).appendingPathComponent("DM1", isDirectory: true)
print("Saving DB backup for DM1")
if !fm.fileExists(atPath: dir.path) {
do {
// Create a directory
try fm.createDirectory(at: dir, withIntermediateDirectories: true, attributes: nil)
let backupURL = dir.appendingPathComponent(url.lastPathComponent)
try fm.copyItem(at: url, to: backupURL)
}
catch {
print("Failed to save DB backup")
}
}
Oops, I need to make another change...
If you run your migration to DM2 then realize you need to make another change, you'll want to re-test your migration from DM1 -> DM2. This is where the backup comes in.
Same way you made the backup, run this code.
let fm = FileManager.default
let url = // The store URL you would use to add the store
let dir = url.deleteLastPathComponent().appendingPathComponent("Backup", isDirectory: true).appendingPathComponent("DM1", isDirectory: true)
let backupURL = dir.appendingPathComponent(url.lastPathComponent)
if fm.fileExists(atPath: backupURL.path) {
do {
fm.removeItem(at: url.path)
try fm.copyItem(at: backupURL, to: url)
}
catch {
print("Failed to restore DB backup")
}
}
You now have a restored DM1 store and have made changes to DM2. If you run the app the migration might succeed but it won't use your custom mapping model.
Remember if you are using a custom mapping, you will still need to use the Refresh Data Models technique before the mapping model will work.
