CoreData Migration (Versioning of CoreData)

iOS, Mobility, Technology

18 / Mar / 2015 by Ashu Baweja 1 comments

What is Migration ?

Migration is a way of telling Xcode how to transition the data from the old model to the new model .

Why migration required ?

When the model does not match the store, a migration is required. In order to perform a migration, Core Data (technically, an instance of NSMigrationManager) requires these things:

The destination managed object model (the one with the changes)
A managed object model that can open the existing store
The ability to infer mapping between the two models (for a lightweight migration), or a manual mapping model
Permission to attempt to perform an automatic migration (for a lightweight migration)

It is therefore absolutely essential that you never make changes to the managed object model for a released version of an app. That is, if your app is already in the App Store, don’t change a single thing in that version of the managed object model.

Solution for the Crash while update an application from the app store:

Create a new version of the managed object model! This reminds me to mention some other best practices to adopt when working with Core Data:

Create a new model version for every release version of an app
Keep a copy of every release version of an app (you’re already doing this, right?)
Keep a copy of every release version backing SQLite store containing suitable test data

Migrations can handle the following changes:

Adding or removing an entity, attribute, or relationship
Making an attribute non-optional with a default value
Making a non-optional attribute optional
Renaming an entity or attribute using a renaming identifier

Getting Started:

Create a new version of the managed object model – select .xcdatamodeld file in Xcode and then from the menu select Editor\Add Model Version

Set version name and select model based on which you want to create new model.

Switch to latest version of model – select .xcdatamodeld file from Project Navigator and go to FILe Inspector and select latest core data model

Now you will see a little green circle with white checkmark badge on latest .xcdatamodel in the Project Navigator.

Add permissions to attempt to perform an automatic migration

Open .m file containing core data related code and add the following code immediately after
persistentStoreCoordinator = [[NSPersistentStoreCoordinator alloc] initWithManagedObjectModel:[self managedObjectModel]];

[code language=”objc”]
NSDictionary *options = @{
NSMigratePersistentStoresAutomaticallyOption : @YES,
NSInferMappingModelAutomaticallyOption : @YES
};
[/code]

Change the next line to pass the options dictionary you just created to
addPersistentStoreWithType:configuration:URL:options:error:

[code language=”objc”]
if (![_persistentStoreCoordinator addPersistentStoreWithType:NSSQLiteStoreType
configuration:nil
URL:storeURL options:options error:&error]) {
[/code]

Now make changes to an core data entity, attribute, or relationship whatever you want

Delete CoreData NSManagedObect Subclass for entities in which you have made changes and create new file (New File/ CoreData/ NSManagedObect)