Core Data Migrations and A Better Option

Core Data Migrations
and …can we do
better?
Priya Rajagopal
@rajagp

Core Data Stack
3
Managed Object
Model (schema)
Persistent Store Coordinator
Managed Object Context (1 ..*)
Managed
Object
Managed
Object
Managed
Object
Persistent
Object Store
(1..*)
Persistent
Store (1..*)

Core Data Stack : iOS10
4
Persistent Container

Managed Object Model
5
• Schema Definition
• Entities , Attributes and Relationships
• Versioning
Image source: https://developer.apple.com/

The Dreaded Persistent Store / Data Model Mismatch
Exception!
7

Option 1 : Delete the app and reinstall
8

Option 2: Core Data Migrations
10
Managed Data Model
(Source)
Managed Data Model
(Target)
Source Data
Store
Mapping Model
Target Data
StoreMigration Policy

Case 1
11
Confidential and Proprietary. Do not distribute without Couchbase consent. © Couchbase 2017. All rights reserved.
Source Model
country domain name
University Target Model
country domain name
University
websiteweblink

Lightweight Migrations
12
• Automatic Migrations
• Core Data infers mapping model from source to target model
• Fast
• Handles following Changes
• Simple addition /removal of a new attribute
• A non-optional attribute becoming optional
• An optional attribute becoming non-optional, and defining a default value
• Renaming an entity or property
• Simple Relationship changes
• Simple entity hierarchy changes
• Complete list
https://developer.apple.com/library/content/documentation/Cocoa/Conceptual/CoreDataVersioning/Articles/vmLightweightMigration.html#//apple_ref/doc/uid/TP40004399-CH4-
SW2

Configure Core Data Stack
13
let persistentStoreDesc = NSPersistentStoreDescription(url: storeUrl)
persistentStoreDesc.shouldMigrateStoreAutomatically = true
persistentStoreDesc.shouldInferMappingModelAutomatically = true
self.persistentContainer?.persistentStoreDescriptions = [persistentStoreDesc]

Case 2
15
Source Model
country domain name
University
website
estDate
(Date)
country domain name website
estYear
(String)

Custom Migration Policy
16
// Transform estDate of type Date to estYear of type String
class UniversityDataMigrationPolicy: NSEntityMigrationPolicy {
func transformEstDateToEstYear(_ estDate:NSDate)->String {
let calendar = NSCalendar.current
let components = calendar.dateComponents(Set<Calendar.Component>([.year]), from: estDate as
Date)
return "(components.year!)"
}
}

Case 3
19
Source Model
country domain name
University
website estYear
name
domain name website estYear
code
country
country
*..1

Custom Migration Policy (1/3)
20
class UniversityMigrationPolicy: NSEntityMigrationPolicy {
override func createDestinationInstances(forSource sInstance: NSManagedObject, in mapping:
NSEntityMapping, manager: NSMigrationManager) throws {
// 1. Create destination Instance corresponding to the source instance
let dInstance = NSEntityDescription.insertNewObject(forEntityName: mapping.destinationEntityName!,
into: manager.destinationContext)
let sKeys = sInstance.entity.attributesByName.keys
let sValues = sInstance.dictionaryWithValues(forKeys: Array(sKeys))
let dKeys = dInstance.entity.attributesByName.keys
for dKey in dKeys {
if let matchingValueInSrc = sValues[dKey] {
dInstance.setValue(matchingValueInSrc, forKey: dKey)
}
}

21
let universityName = sValues["name"]
if let countryName = sInstance.value(forKey: "country") as? String {
// 2 . We are tracking the Country managed objects in Migration Manager’s userInfo
// This is to ensure we create a single instance of Country for every countryName
if let userInfo = manager.userInfo , let country = userInfo[countryName] as? NSManagedObject{
// link university entity to country
dInstance.setValue(country, forKey: "country")
}
else {

22
// 3 . Create Country object
let country = NSEntityDescription.insertNewObject(forEntityName: "Country", into: manager.destinationContext)
// Set country name
country.setValue(countryName, forKey: "name")
// 4. Link University object with the corresponding Country object
dInstance.setValue(country, forKey: "country")
// 5. Track the newly created country object in Migration Manager’s userInfo
if let _ = manager.userInfo {
manager.userInfo?[countryName] = country
}
else {
manager.userInfo = [:]
manager.userInfo?[countryName] = country
}
}
// 4. Associate the source with destination instance
manager.associate(sourceInstance: sInstance, withDestinationInstance: dInstance, for: mapping)
}

Manual
24
let persistentStoreDesc = NSPersistentStoreDescription(url: storeUrl)
persistentStoreDesc.shouldMigrateStoreAutomatically = false
persistentStoreDesc.shouldInferMappingModelAutomatically = false
self.persistentContainer?.persistentStoreDescriptions = [persistentStoreDesc]
• Create Custom Migration Policies
• Create Mapping Models

1. Test if Migration is Needed
25
func isMigrationNeeded()->Bool {
……
let srcMetadata = try NSPersistentStoreCoordinator.metadataForPersistentStore(ofType: NSSQLiteStoreType, at: sourceUrl,
options: nil)
let momPathValue = Bundle.main.path(forResource: "UniversityDataModel", ofType:"momd")
guard let destModel = NSManagedObjectModel(contentsOf: URL(fileURLWithPath: momPathValue)) else { return false }
return destModel.isConfiguration(withName: nil, compatibleWithStoreMetadata: srcMetadata) == false ? true: false
……
}

2. Migrate Stores Progressively
26
func progressivelyMigrate() throws {
// Get the model version that is compatible with the current data store
let idx = try indexOfCompatibleMom(at: sourceUrl, moms: self.models)
let remaining = self.models.suffix(from: (idx + 1))
guard remaining.count > 0 else {
// The stored DB is compatible with the latest model
return // migration not necessary
}
_ = try remaining.reduce(self.models[idx]) { smom, dmom in
do {
try self.migrate(srcStoreUrl: sourceUrl, srcModel: smom, dstModel: dmom)
}
catch {
// handle error
}
return dmom
}
}

3. Identify Model compatible with Store
27
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
}

4. Migrate Store from src to dst model
28
func migrate(srcStoreUrl:URL, srcModel:NSManagedObjectModel,dstModel:NSManagedObjectModel)throws {
// Prepare temp directory for destination
let dir = 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: srcModel, to: dstModel)
let destURL = dir.appendingPathComponent(srcStoreUrl.lastPathComponent)
let manager = NSMigrationManager(sourceModel: srcModel, destinationModel: dstModel)
try autoreleasepool {
try manager.migrateStore(
from: srcStoreUrl,
sourceType: NSSQLiteStoreType,
options: nil,
with: mapping,
toDestinationURL: destURL,
destinationType: NSSQLiteStoreType,
destinationOptions: nil
)
}

5. Find Mapping Model
29
fileprivate 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 inferred mapping model if a custom model not defined.
return try NSMappingModel.inferredMappingModel(forSourceModel: smom, destinationModel: dmom)
}

Core Data Migrations and A Better Option

NoSQL Option for Persisting Data
31
• Non Relational
• Unstructured or Semi Structured Data
• Scalable – both up and down
• SQL-type Queries
Document
• Couchbase
• MongoDB
• DynamoDB
• DocumentDB
Key-Value
• Couchbase
• Riak
• BerkeleyDB
• Redis
• …
Graph
• OrientDB
• Neo4J
• DEX
• GraphBase
Wide Column
• Hbase
• Cassandra
• Hypertable

Data Modeling
32
Data Concern Core Data JSON Document Model (Couchbase)
Rich Structure  Managed Object Model - entities  Documents = containers of related KV pairs
Relationships  Represented  Represented (embed/reference)
Structure Evolution  Migrations
 Flexible
 Dynamic change
Value Evolution  Data can be updated  Data can be updated
Query Interface
 NSFetchRequest
 NSFetchedResultsControllerDelegate
 Query / LiveQuery

NoSQL: Example 1
US Umich.edu
University Of
Michigan US harvard.edu Harvard University -10516645301
{
"type":"university",
"name":"University of Michigan",
"domain":"umich.edu",
"country":"US”
}
{
"name":"Harvard University”,
"domain":"harvard.edu",
"country":"US",
"estDate":-10516645301
}

NoSQL : Example 2
US harvard.edu
Harvard
University
{
"name":"Harvard University",
"domain":"harvard.edu”,
"country":"US",
"estDate":-10516645301
}
-10516645301
(Double)
US harvard.edu Harvard University “1636”
(String)
{
"name":"Harvard University",
"domain":"harvard.edu”,
"country":"US",
"estYear": “1636”
}
let estDate = university.double(forKey: "estDate")
if estDate != 0.0 {
// Handle estDate
}else if let estYear = university.string(forKey: "estYear") {
// Handle estYear
}

A NoSQL Option for iOS :
A Sneak Peek

Couchbase Mobile Platform: Full Stack Database Platform
37
Couchbase Lite Sync Gateway Couchbase Server
EMBEDDED DATABASE SYNCHRONIZATION DATABASE SERVER
SECURITY
NoSQL Document Style

Couchbase Lite
38
Mobile App
Couchbase Lite
Native API
Native Platform
Bindings
JSON
Lite Core Engine
Fleece
SQLite Engine
• Embedded NoSQL Database
• JSON Document Store
• Open Source
• iOS, Android, .NET…
• Encryption Support
• Powerful Query Interface, FTS
• Sync (full Couchbase stack)
• Conflict Resolver

Resources
39
• Couchbase Developer Portal
• https://developer.couchbase.com
• Source Code
• https://github.com/couchbaselabs
• https://github.com/couchbase/
• “Office Hours”

Integrating into your iOS Project
40
developer.couchbase.com

Creating / Opening a Database
41
// Set Database configuration
var config = DatabaseConfiguration()
config.directory = userFolderPath
config.encryptionKey( /*encryption key object*/ )
config. conflictResolver(/*Set custom resolver */ )
// Create / Open a database with specified name and configuration
_db = try Database(name: kDBName, config: options)

Inserting Document
42
// Create a new document or update existing document
// If Id is not provided, system generates Id
let document = Document.init(id, dictionary: props)
try database.save(document)
Supported Data Types :
• Date
• Number
• Null
• String
• Array
• Blob
• Dictionary
Typed Accessors

Live Queries (w/ Full Text Search)
let query = Query.select(SelectResult.all())
.from(DataSource.database(database))
.where((Expression.property("skills").in(["music","writing"]))
.and(Expression.property("type").match(”student")))
.orderBy(Ordering.property("name").ascending()). toLive();
// Register for live query changes
_liveQueryListener = _bookingQuery?.addChangeListener({ [weak self](change) in
for (_, row) in (change.rows?.enumerated())! {
// handle row.document
default:
}})
query.run();

Core Data Migrations and A Better Option

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