Manual local migration for AWS DataStore

DataStoreMigrationManager.swift

import SQLite

// Check that there is no other migration in progress and local DB files are present
// Backup the DB just in case
// ...

// URL to the local DB copy
let dataStoreLocalCopyURL = ...

do {
  let decoder = JSONDecoder()
  let db = try Connection(dataStoreLocalCopyURL.absoluteString)
  let table = Table("MutationEvent")
  let json = Expression<String>("json")
  let modelName = Expression<String>("modelName")
  let mutationType = Expression<String>("mutationType")
  let preparedDB = try db.prepare(table)

  var eventsCount = 0

  let eventsForMigration: [String : Model.Type] = [
    String(describing: ModelName.self) : ModelName.self,
    // List of all models to migrate
    // ...
  ]

  for mutationEvent in preparedDB {
    let json = mutationEvent[json]
    let mutationType = MutationType(rawValue: mutationEvent[mutationType]) ?? .unknown
    let modelName = mutationEvent[modelName]

    guard let jsonData = json.data(using: .utf8) else {
      fatalError("Invalid model")
      continue
    }

    guard let event = eventsForMigration[modelName] else {
      fatalError("Event does not support Migration")
      continue
    }

    eventsCount += 1

    let parsedEvent = try decoder.decode(event, from: jsonData)

    switch mutationType {
    case .create, .update:
      AmplifyClient.save(model: parsedEvent)
    case .delete:
      AmplifyClient.delete(model: parsedEvent)
    default:
      break
    }
  }
} catch {
  fatalError("Migration failed")
}