Initialization of Core Data stack that needs a manual migration from 2 different models and stores:

gistfile1.txt

// see also this gist: https://gist.github.com/horseshoe7/e85bbb90278f626bea2f827ee5228703
// and a post about Core Data migrations at http://horseshoe7.wordpress.com

import Foundation
import CoreData

class CoreDataStackHelper : NSObject, MHWMigrationManagerDelegate {
    
    var currentModelStoreURL: URL? = nil
    
    func initializePersistentContainer(completion: ((_ container: NSPersistentContainer) -> Void)?)  {
        
        let container = NSPersistentContainer(name: "SongbookSimpleDataModel") // your newest model you want to migrate your legacy store to!
        
        assert(container.persistentStoreDescriptions.count <= 1, "Never thought there would be more than one Persistent Store Description for this data model!")
        for pscDescription in container.persistentStoreDescriptions {
            self.currentModelStoreURL = pscDescription.url
        }
        
        // here you need to work on a "Migration Stack"
        if self.requiresLegacyCoreDataMigration() {
            
            do {
                try self.migrateLegacyModelToCurrent(using: container)
                self.migrationDidComplete()
            } catch {
                
            }
        }

        container.loadPersistentStores(completionHandler: { (storeDescription, error) in
            if let error = error as NSError? {
                // Replace this implementation with code to handle the error appropriately.
                // fatalError() causes the application to generate a crash log and terminate. You should not use this function in a shipping application, although it may be useful during development.
                
                /*
                 Typical reasons for an error here include:
                 * The parent directory does not exist, cannot be created, or disallows writing.
                 * The persistent store is not accessible, due to permissions or data protection when the device is locked.
                 * The device is out of space.
                 * The store could not be migrated to the current model version.
                 Check the error message to determine what the actual problem was.
                 */
                
                
                fatalError("Unresolved error \(error), \(error.userInfo)")
            }
            
            // we add this so that any background work gets merged!
            container.viewContext.automaticallyMergesChangesFromParent = true
            
            completion?(container)
        })
    }
    
    fileprivate func migrationDidComplete() {
        
        let storeURL = self.legacyCoreDataStoreURL
        let storeURLDirectory = storeURL.deletingLastPathComponent()
        var isDir = ObjCBool(false)
        if FileManager.default.fileExists(atPath: storeURLDirectory.path, isDirectory: &isDir) {
            
            if isDir.boolValue {
                do {
                    try FileManager.default.removeItem(atPath: storeURLDirectory.path)
                } catch {
                    
                }
            }
        }
    }
    
    // this is custom to your application!
    fileprivate var legacyCoreDataStoreURL: URL {
        
        // I had to install and old version then inspect the file structure.  This is very specific to your app
        if let documentsURL = FileManager.default.urls(for: .applicationSupportDirectory, in: .userDomainMask).last {
            var storeURL = documentsURL.appendingPathComponent("SongbookSimpleCoreData/")
            storeURL = storeURL.appendingPathComponent("SongbookSimple.sqlite")
            return storeURL
        }
        fatalError("You are fetching the url incorrectly.  This method should never fail")
    }
    
    fileprivate func requiresLegacyCoreDataMigration() -> Bool {
        
        return FileManager.default.fileExists(atPath: self.legacyCoreDataStoreURL.path)
    }
    
    // this is mostly custom to your application
    fileprivate func migrateLegacyModelToCurrent(using container: NSPersistentContainer) throws {
        
        let momdName = "SongbookSimple" //pass this as a parameter.  Legacy Model Name
        
        guard let modelURL = Bundle(for: type(of: self)).url(forResource: momdName, withExtension:"momd") else {
            fatalError("Error loading model from bundle")
        }
        
        guard let _ = NSManagedObjectModel(contentsOf: modelURL) else {
            fatalError("Error initializing mom from: \(modelURL)")
        }
        
        
        let storeURL = self.legacyCoreDataStoreURL
        
        let migrationManager = MHWMigrationManager()
        migrationManager.delegate = self
        let migrationOptions: [AnyHashable: Any]? = nil
        
        do {
            try migrationManager.progressivelyMigrateURL(storeURL, ofType: NSSQLiteStoreType, options: migrationOptions, to: container.managedObjectModel)
        } catch {
            log.error("Error During Migration: \(error.localizedDescription)")
            throw error
        }
    }
    
    func destinationURLOfFullyMigratedStoreRequested(by migrationManager: MHWMigrationManager) -> URL? {
        return self.currentModelStoreURL
    }
    
    func migrationManager(_ migrationManager: MHWMigrationManager, migrationProgress: Float) {
        log.info(String(format:"Migration Progress: %.2f", migrationProgress))
    }
    
    // this is to help the migration manager if it can't resolve these on its own
    func migrationManager(_ migrationManager: MHWMigrationManager, mappingModelsForSourceModel sourceModel: NSManagedObjectModel) -> [Any] {
        
        var mappingModels: [NSMappingModel] = []
        guard let modelName = sourceModel.mhw_modelName() else {
            log.error("Could not derive the model name from the source model!")
            return []
        }
        
        if modelName == "SongbookSimple 3" {
            // migrate old store to old store
            if let urls = Bundle(for: type(of: self)).urls(forResourcesWithExtension: "cdm", subdirectory: nil) {
                for url in urls {
                    if let foundRange = url.lastPathComponent.range(of: "Migration_3-to_4"), foundRange.isEmpty == false {
                        if let mappingModel = NSMappingModel(contentsOf: url) {
                            if let userRange = url.lastPathComponent.range(of: "User"), userRange.isEmpty == false {
                                mappingModels.insert(mappingModel, at: 0)
                            } else {
                                mappingModels.append(mappingModel)
                            }
                        }
                    }
                }
            }
        } else if modelName == "SongbookSimple 4" {
            // migrate old model to new model
            if let urls = Bundle(for: type(of: self)).urls(forResourcesWithExtension: "cdm", subdirectory: nil) {
                for url in urls {
                    if let foundRange = url.lastPathComponent.range(of: "Migration-Legacy-To-Modern"), foundRange.isEmpty == false {
                        if let mappingModel = NSMappingModel(contentsOf: url) {
                            if let userRange = url.lastPathComponent.range(of: "User"), userRange.isEmpty == false {
                                mappingModels.insert(mappingModel, at: 0)
                            } else {
                                mappingModels.append(mappingModel)
                            }
                        }
                    }
                }
            }
        }
        return mappingModels
    }
}