Core Data Progressive Migration

Migration.swift

// The MIT License (MIT)
//
// Copyright (c) 2016 Alexander Grebenyuk (github.com/kean).

import Foundation
import CoreData

enum MigrationError: ErrorProtocol {
    case IncompatibleModels
}

// moms: [mom_v1, mom_v2, ... , mom_vN]
func migrateStore(at storeURL: URL, moms: [NSManagedObjectModel]) throws {
    let idx = try indexOfCompatibleMom(at: storeURL, moms: moms)
    let remaining = moms.suffix(from: (idx + 1))
    guard remaining.count > 0 else {
        return // migration not necessary
    }
    _ = try remaining.reduce(moms[idx]) { smom, dmom in
        try migrateStore(at: storeURL, from: smom, to: dmom)
        return dmom
    }
}

func indexOfCompatibleMom(at storeURL: URL, moms: [NSManagedObjectModel]) throws -> Int {
    let meta = try NSPersistentStoreCoordinator.metadataForPersistentStore(ofType: NSSQLiteStoreType, at: storeURL)
    guard let idx = moms.index(where: { $0.isConfiguration(withName: nil, compatibleWithStoreMetadata: meta) }) else {
        throw MigrationError.IncompatibleModels
    }
    return idx
}

func migrateStore(at storeURL: URL, from smom: NSManagedObjectModel, to dmom: NSManagedObjectModel) throws {
    // Prepare temp directory
    let dir = try URL(fileURLWithPath: NSTemporaryDirectory()).appendingPathComponent(UUID().uuidString)
    try FileManager.default().createDirectory(at: dir, withIntermediateDirectories: true, attributes: nil)
    defer {
        _ = try? FileManager.default().removeItem(at: dir)
    }

    // Perform migration
    let mapping = try findMapping(from: smom, to: dmom)
    let destURL = try dir.appendingPathComponent(storeURL.lastPathComponent!)
    let manager = NSMigrationManager(sourceModel: smom, destinationModel: dmom)
    try autoreleasepool {
        try manager.migrateStore(
            from: storeURL,
            sourceType: NSSQLiteStoreType,
            options: nil,
            with: mapping,
            toDestinationURL: destURL,
            destinationType: NSSQLiteStoreType,
            destinationOptions: nil
        )
    }

    // Replace source store
    let psc = NSPersistentStoreCoordinator(managedObjectModel: dmom)
    try psc.replacePersistentStore(
        at: storeURL,
        destinationOptions: nil,
        withPersistentStoreFrom: destURL,
        sourceOptions: nil,
        ofType: NSSQLiteStoreType
    )
}

func findMapping(from smom: NSManagedObjectModel, to dmom: NSManagedObjectModel) throws -> NSMappingModel {
    if let mapping = NSMappingModel(from: Bundle.allBundles(), forSourceModel: smom, destinationModel: dmom) {
        return mapping // found custom mapping
    }
    return try NSMappingModel.inferredMappingModel(forSourceModel: smom, destinationModel: dmom)
}