Lightweight Migrations in Core Data Tutorial

Learn how to keep your data models up-to-date with this Core Data migrations tutorial! By Aaron Douglas.

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This is an abridged chapter from our book Core Data by Tutorials, which has been completely updated for Swift 4.2 and iOS 12. This tutorial is presented as part of our iOS 12 Launch Party — enjoy!

During app development, well before the ship date, thorough testing can help iron out the data model. However, changes in app usage, design or features after an app’s release will inevitably lead to changes in the data model. What do you do then?

You can’t predict the future, but with Core Data, you can migrate toward the future with every new release of your app. The migration process will update data created with a previous version of the data model to match the current data model.

This tutorial discusses the many aspects of Core Data migrations by walking you through the evolution of a note-taking app’s data model. You’ll start with a simple app with only a single entity in its data model.

Let the great migration begin!

When to Migrate

When is a migration necessary? The easiest answer to this common question is “when you need to make changes to the data model.”

However, there are some cases in which you can avoid a migration. If an app is using Core Data merely as an offline cache, when you update the app, you can simply delete and rebuild the data store. This is only possible if the source of truth for your user’s data isn’t in the data store. In all other cases, you’ll need to safeguard your user’s data.

That said, any time it’s impossible to implement a design change or feature request without changing the data model, you’ll need to create a new version of the data model and provide a migration path.

The Migration Process

When you initialize a Core Data stack, one of the steps involved is adding a store to the persistent store coordinator. When you encounter this step, Core Data does a few things prior to adding the store to the coordinator. First, Core Data analyzes the store’s model version. Next, it compares this version to the coordinator’s configured data model. If the store’s model version and the coordinator’s model version don’t match, Core Data will perform a migration, when enabled.

Note: If migrations aren’t enabled, and the store is incompatible with the model, Core Data will simply not attach the store to the coordinator and specify an error with an appropriate reason code.

To start the migration process, Core Data needs the original data model and the destination model. It uses these two versions to load or create a mapping model for the migration, which it uses to convert data in the original store to data that it can store in the new store. Once Core Data determines the mapping model, the migration process can start in earnest.

Migrations happen in three steps:

  1. First, Core Data copies over all the objects from one data store to the next.
  2. Next, Core Data connects and relates all the objects according to the relationship mapping.
  3. Finally, enforce any data validations in the destination model. Core Data disables destination model validations during the data copy.

You might ask, “If something goes wrong, what happens to the original source data store?” With nearly all types of Core Data migrations, nothing happens to the original store unless the migration completes without error. Only when a migration is successful, will Core Data remove the original data store.

Types of Migrations

In my own experience, I’ve found there are a few more migration variants than the simple distinction between lightweight and heavyweight migrations that Apple calls out. Below, I’ve provided the more subtle variants of migration names, but these names are not official categories by any means. You’ll start with the least complex form of migration and end with the most complex form.

Lightweight Migrations

Lightweight migration is Apple’s term for the migration with the least amount of work involved on your part. This happens automatically when you use NSPersistentContainer, or you have to set some flags when building your own Core Data stack. There are some limitations on how much you can change the data model, but because of the small amount of work required to enable this option, it’s the ideal setting.

Manual Migrations

Manual migrations involve a little more work on your part. You’ll need to specify how to map the old set of data onto the new set, but you get the benefit of a more explicit mapping model file to configure. Setting up a mapping model in Xcode is much like setting up a data model, with similar GUI tools and some automation.

Custom Manual Migrations

This is level 3 on the migration complexity index. You’ll still use a mapping model, but complement that with custom code to specify custom transformation logic on data. Custom entity transformation logic involves creating an NSEntityMigrationPolicy subclass and performing custom transformations there.

Fully Manual Migrations

Fully manual migrations are for those times when even specifying custom transformation logic isn’t enough to fully migrate data from one model version to another. Custom version detection logic and custom handling of the migration process are necessary. In this tutorial, you’ll set up a fully manual migration to update data across non-sequential versions, such as jumping from version 1 to 4.

Throughout this tutorial, you’ll learn about each of these migration types and when to use them. Let’s get started!

Getting Started

Download the resources for this tutorial using the “Download materials” button Included with the resources for this tutorial is a starter project called UnCloudNotes. Find the starter project and open it in Xcode.

Build and run the app in the iPhone simulator. You’ll see an empty list of notes:

Tap the plus (+) button in the top-right corner to add a new note. Add a title (there’s default text in the note body to make the process faster) and tap Create to save the new note to the data store. Repeat this a few times so you have some sample data to migrate.

Back in Xcode, open the UnCloudNotesDatamodel.xcdatamodeld file to show the entity modeling tool in Xcode. The data model is simple — just one entity, a Note, with a few attributes.

You’re going to add a new feature to the app: the ability to attach a photo to a note. The data model doesn’t have any place to persist this kind of information, so you’ll need to add a place in the data model to hold onto the photo. But you already added a few test notes in the app. How can you change the model without breaking the existing notes?

It’s time for your first migration!