Use Swift 3 This is recommended. Migrations are an important aspect of Core Data because most applications grow and that often means you need to make changes to the data model. It's time to create a new version of the data model. Design like a professional without Photoshop. What we haven't told Core Data is what it should do if it runs into an incompatibility issue. However, because we changed the data model by adding an attribute to the Item entity, updatedAt, Core Data no longer understands how it should store Item records in the SQLite database. Before we make any changes, select Lists.xcdatamodeld (not Lists.xcdatamodel), open the File Inspector on the right, and set Model Version to Lists 2, the data model version we created a moment ago. The latter is very useful for debugging the issue. This should feel familiar by now. This makes it tempting to creat… Versioning and migrations go hand in hand. Note that I've changed the value of URLPersistentStore, the location of the persistent store. If you don't invest time architecting the data model, chances are you run into problems that could have been avoided. We show an alert to the user, but it's a good idea to take it a few steps further. In all other cases, you’ll need to safeguard your user’s data. In this tutorial, we take a look at how Core Data helps us manage changes of the data model and what pitfalls we absolutely need to avoid. With the new data model in place, we ran the application again. Add two attributes to the User entity: Add a relationship, lists, to the User entity and set its destination to the List entity. I have never come in a situation that warranted calling abort in production and it pains me when I browse a project in which Apple's default implementation for setting up the Core Data stack is used, in which abort is called when adding a persistent store is unsuccessful. The Guide I Wish I Had When I Started Out, Join 20,000+ Developers Learning About Swift Development. If you wish to understand how migrations work, you'll first need to understand how to version the Core Data data model. Adding or removing entities, attributes, and relationships are no problem for Core Data. Design, code, video editing, business, and much more. In other words, we need to hand Core Data the necessary information to migrate the persistent store from one version of the data model to another. When Core Data loads a persistent store from disk, it resolves the matching hashes in the persistent store against the MOM objects included with the application. With the data model selected, choose Add Model Version... from the Editor menu. You should never modify a data model without telling Core Data about the changes you made. If you change the cardinality of a relationship, then you are in for a wild ride. To solve that problem, we first and foremost leave the data model that was used to create the persistent store untouched. The goal of this article is to give you an overview of the Core Data architecture as well as to provide a list of basic operations together with working examples to help you get off to a quick start.. Heavy migrations require a lot of work and testing to make sure the migration completes successfully and, more importantly, without data loss. Add a method to the subclass to convert the value. You can download the source files of the tutorial from GitHub. It's time to verify that everything is still working. Remember that the inverse relationship of the lists relationship is automatically set for us by Core Data. If you plan to modify relationships or change attribute types, then you're in for a wild ride with heavy migrations. EF Core compares the current model against a snapshot of the old model to determine the differences, and generates migration source files; the files can be tracked in your project's source control like any … The implementation of applicationStoresDirectory(), a helper method, is straightforward as you can see below. Trademarks and brands are the property of their respective owners. Core Data tells us that the data model is not compatible with the data model we used to create the persistent store. When it comes to structured data persistence, Core Data is an obvious choice. What? The changes you can make to a data model with lightweight migrations are limited. That's correct, versioning the data model is part of the solution. You should see the two versions of the data model, Done.xcdatamodel and Done 2.xcdatamodel. Questions? When we launched the application for the first time, Core Data created a SQLite database based on the data model. The assistant can also be invoked manually from the menu Edit -> Convert -> To Current Swift Syntax… You can choose from two kinds of migration to perform: Use Swift 2.3 Modifies your project to enable the Use Legacy Swift build setting and provides source changes to be able to build against the new SDKs. Heavy migrations are a pain in the neck and you should try to avoid them if possible. If the data model changes, we need to tell Core Data how to migrate the data of the persistent store to the new data model version. In the Data Model Inspector, set Type to To Many. The second key, NSInferMappingModelAutomaticallyOption, instructs Core Data to infer the mapping model for the migration. If adding the persistent store to the persistent store coordinator fails, the application invokes the fatalError() function, which causes the application to terminate immediately. There are two types of migrations, lightweight and heavy migrations. In the development phase of a project we keep on adding new features in every versions of app. Swift – Core Data – Part 3 Creating a Singleton Core Data & refactoring insert, update, delete operations; Swift – Core Data — Part 4 Writing unit test cases; Swift – Core Data — Part 5 Core Data Migration; Swift – Core Data — Part 6 Using NSFetchResultController with UITableView Read this paragraph again. If you've worked with Ruby on Rails or any other framework that supports migrations, then Core Data migrations will make a lot of sense to you. Versioning and migrations go … Hello readers. The cost of a lightweight migration is substantially lower than that of a heavy migration. Core Data Migration In order to follow core data migration we should keep on versioning our.xcdatamodeld file. pyrabbit / new_core_data_stack.swift. Select the List entity and create a To One relationship with the User entity as its destination. Star 0 Fork 0; Code Revisions 1. It is fine to make incremental changes to the data model as your application grows, but once your application is in the hands of users you need to make sure they don't lose their data due to a problematic migration. This indicates that Lists.xcdatamodel is the active data model version. It is also called light weight migration. That's what happens if you mess with the data model without telling Core Data about it. Note that there are a few caveats you should be aware of. In the next article, we focus on subclassing NSManagedObject. CoreData Swift: How to save and load data? The words lightweight and heavy are pretty descriptive, but it's important that you understand how Core Data handles each type of migration. Note that the green checkmark has moved from Lists.xcdatamodel to Lists 2.xcdatamodel. If you do run into this issue, then double-check the value of URLPersistentStore. It gives us the opportunity to tell the persistent store coordinator how it should migrate the persistent store that we want to add to it. 2) After that, core data finds relates and connects all the objects according to the relationship mapping. And you know what happened next. Do you remember that addPersistentStore(ofType:configurationName:at:options:) accepts a dictionary of options as its last parameter? This dictionary of options includes instructions for Core Data. In the Project Navigator, Done 2.xcdatamodel should now have a green checkmark instead of Done.xcdatamodel. Run the application again. Before a persistent store is added to the persistent store coordinator, Core Data checks if a persistent store already exists. Basically the Core data migration happens in three steps: 1) Core data copies all objects from initial data store to the next. If Core Data was unable to migrate the persistent store using the data model, we remember this by setting a key-value pair in the application's user defaults database. The implementation of applicationIncompatibleStoresDirectory() is similar to that of applicationStoresDirectory(). Now that you know what Core Data is and how the Core Data stack is set up, it's time to write some code. That said, any time it’s impossible to imple… Note that we remove the key-value pair when the user taps the button of the alert. If Core Data tells us the data model and persistent store are incompatible, then it's up to us to resolve that. We then modified the data model by creating the User entity and defining several attributes and relationships. How do you know if a data model change requires a lightweight or a heavyweight migration? It's time to create a new persistent store to finish the setup of the Core Data stack. Lightweight migrations are much easier because Core Data takes care of the heavy lifting for us. I cannot stress enough how important this phase of a project is. It points to a directory in the Documents directory in the application's sandbox. To make changes to the data model, we've added a new data model version. If the matching MOM isn’t flagged as the “current” MOM, data migration will then kick in. It's the only option available. Run the application in the simulator or on a physical device to make sure everything is set up correctly. Created Mar 22, 2017. Run the application again and inspect the output in the console. Have you noticed in the Project Navigator that one of the versions has a green checkmark? Before we version the data model, we need to revert the data model to its original state. Hope you are doing good. You can verify this by right-clicking the .xcdatamodeld file and selecting Show in Finder. Core Data is Apple’s object graph management and persistency framework. With this change, you can safely build and run the application. Data migration can be a headache. For example, a lightweight migration lets you add or rename attributes and entities, but you cannot modify the type of an attribute or the relationships between existing entities. This step is probably the most important one in terms of creating a user friendly application. Ligheweight Coredata migration is the easiest and simplest way of migrating data models with least possible efforts. Entries like $source.timestamp mean to copy the existing value from before the migration. In 2014, the research and advisory company Gartner revealed that a significant percentage of data migration projects go over budget, cause business disruption or fail altogether. Run the application in the simulator or on a physical device to make sure everything is set up correctly. Give the subclass an obvious name like ModelMigration1to2. The application should run and work just fine. By looking at the versioned data model, it also knows if the persistent store needs to be migrated before it can be used with the current version of the data model. Contribute to tutsplus/CoreDataSwift-Migrations development by creating an account on GitHub. Click the triangle to show the list of data model versions. Take a look at the updated implementation of the persistentStoreCoordinator property in AppDelegate.swift. The idea is simple. Migrations are an important topic if you plan to make extensive use of Core Data. A mapping model defines how one version of the data model relates to another version. Download or clone the projectwe created in the previous tutorial and open it in Xcode. Each entity version in each data model has a unique hash. Create a new subclass of NSEntityMigrationPolicy. Name the relationship user and set the inverse relationship to lists. Whenever we modify the data model of a Core Data applicati. Using this method, coredata migrates the data automatically. As we saw earlier, adding an attribute, updatedAt, to the Item entity results in the persistent store being incompatible with the modified data model. Along with Core Data versioning, migration and iCloud integration, I will take you on developing 5 iOS applications and how we can integrate all of these components into Core Data. We need to tell Core Data how to migrate the persistent store for the data model. Core Data allows us to version the data model and this enables us to safely modify the data model. In the previous articles of this series, we've encountered an annoying issue that we need to address. Don't forget to also add the user relationship to the List entity. In nameForIncompatibleStore(), we generate a name for the incompatible store based on the current date and time to avoid naming collisions. Also to address new changes we should not make any changes in existing data model. Master the Core Data in Swift 4.2 and iOS 12, and take your iOS development skills to the next level. This class will tell Core Data how to map the old boolean value to the new integer value. For example, if you've set the data model version to Done 2.xcdatamodel, run the application, and then make changes to Done 2.xcdatamodel, then you'll most likely run into a crash due to the persistent store being incompatible with the data model. Subscribe below and we’ll send you a weekly email summary of all new Code tutorials. In this blog post we will be comparing Core Data with Xcode 7.3.1 vs new Core Data with Xcode 8 beta to see if it has become any easier to use Core Data. Run the application to see if the solution works. The first key, NSMigratePersistentStoresAutomaticallyOption, tells Core Data that we'd like it to attempt to migrate the persistent store for us. I already told you that a persistent store is tied to a particular version of the data model. You always need to test the migration to be sure. A Heavy/Manual Migration A heavy migration is required when we go outside of the bounds of what a light migration can accomplish. We use the latest and greatest to build an application. Open CoreDataManager.swift and inspect the implementation of the persistentStoreCoordinator property. There is one top level data model with a .xcdatamodeld extension and two children with a .xcdatamodel extension. Data migration in core banking is all about the seamless movement of entries, balances, P&L/balance sheet data, customer information, contracts, products, KYC details and other forms of financial/nonfinancial data from the source to the target system. Like two members of the same friend group who don't really know each other but really like their UIKit friend, Core Data and Unit Testing have in fact discovered that they have a lot in common and have gradually got more and more friendly with each other.. Leave them in the comments below or reach out to me on Twitter. Migrations let you safely modify your application's data model and, in the case of lightweight migrations, without much overhead. Instead we should create a new version of.xcdatamodeld and perform changes there. All gists Back to GitHub. In other words, the modified data model is no longer compatible with the persistent store, the SQLite database, it created earlier. Wait. Select Lists.xcdatamodeld and remove the User entity and the user relationship of the List entity. :D. Today we will discuss about core data migration. Developers are only human and we all make mistakes, but that doesn't mean you should throw in the towel and call abort. If Core Data is unable to create a new persistent store, then there are more serious problems that are not related to the data model being incompatible with the persistent store. Still modify the data model and this enables us to resolve that kick start your project! 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