If you wish to understand how migrations work, you'll first need to understand how to version the Core Data data model. If you run into a crash, then you've done something wrong. This should feel familiar by now. Created Mar 22, 2017. Migrations are great, but they should be used sparingly. Questions? We also marked the new data model version as the active data model version. So if you are interested in learning more about core data, please Envato Tuts+ tutorials are translated into other languages by our community members—you can be involved too! If you plan to modify relationships or change attribute types, then you're in for a wild ride with heavy migrations. Select Lists.xcdatamodeld and remove the User entity and the user relationship of the List entity. The latter is very useful for debugging the issue. Run the application to see if the solution works. Select the List entity and create a To One relationship with the User entity as its destination. Trademarks and brands are the property of their respective owners. All gists Back to GitHub. The idea is simple but powerful. 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. In all other cases, you’ll need to safeguard your user’s data. Heavy migrations are powerful, but that power comes at a cost. But how do you tell Core Data about the changes you make to a data model? It points to a directory in the Documents directory in the application's sandbox. Note that we remove the key-value pair when the user taps the button of the alert. Whenever we modify the data model of a Core Data applicati. A small triangle has appeared on the left of the data model in the Project Navigator. Get access to over one million creative assets on Envato Elements. To test our recovery strategy, run the application in the simulator and double-check that the persistent store was successfully created in the Application Support directory, in the Stores subdirectory. Open Done.xcdatamodeld and add an attribute updatedAt of type Date to the Item entity. Core Data Migration In order to follow core data migration we should keep on versioning our.xcdatamodeld file. Run the application one more time and notice how the application crashes as soon as it's launched. We build an application that is powered by Core Data and you learn everything you need to know to use Core Data in your own projects. How this works becomes clear in a moment. The data layer of a software project requires care, attention, and preparation. Core Data migration problem. Question or problem in the Swift programming language: I’m writing an iOS game in Swift, and I wanted to include a highscore label at the end. Add two attributes to the Userentity: 1. firstName of type String 2. lastName of type String Add a relationship, lists, to the User entity and set its destination to the List entity. Open the project's data model by selecting Lists.xcdatamodeld in the Project Navigator. Design like a professional without Photoshop. When you add a new version of your data model, you are asked to select the model on which it should be based. In the previous articles of this series, we've encountered an annoying issue that we need to address. 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. You can verify this by right-clicking the .xcdatamodeld file and selecting Show in Finder. This is what the data model now looks like. 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. However, there is no need to terminate our application, let alone crash it. Using this method, coredata migrates the data automatically. It's time to create a new persistent store to finish the setup of the Core Data stack. Take a look at the following code snippet in which we pass a dictionary of options with two key-value pairs. Heavyweight migrations, however, are a pain. We now understand the cause of this. The Solution. If you're serious about Core Data, check out Mastering Core Data With Swift. 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. With this change, you can safely build and run the application. Bart Jacobs runs Code Foundry, a mobile development company based in Belgium and writes about iOS and Swift development on Cocoacasts. :D. Today we will discuss about core data migration. A simple example of something that goes beyond … - Selection from Core Data in Swift [Book] Swift — 4 — Core Data, We are done with core data migration. Hello readers. Persisting data is an important aspect of most applications. Take a look at the updated implementation of the persistentStoreCoordinator property in AppDelegate.swift. Instead we should create a new version of.xcdatamodeld and perform changes there. 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. Run the application in the simulator or on a physical device to make sure everything is set up correctly. The data model, for example, grows and changes. As we saw earlier, adding an attribute, updatedAt, to the Item entity results in the persistent store being incompatible with the modified data model. The second key, NSInferMappingModelAutomaticallyOption, instructs Core Data to infer the mapping model for the migration. Add an entity to the data model and name it User. The words lightweight and heavy are pretty descriptive, but it's important that you understand how Core Data handles each type of migration. It's time to verify that everything is still working. Because heavy migrations are a complex topic, we won't cover it in this series. That said, any time it’s impossible to imple… Click the triangle to show the list of data model versions. 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. Select Lists 2.xcdatamodel and create the User entity we added earlier. With this in mind, we can update the implementation of the do clause of the do-catch statement in the CoreDataManager class. What? If you do run into this issue, then double-check the value of URLPersistentStore. Host meetups. A mapping model needs a source and a destination data model. NSMigrationManagercan infer the mapping model between two models. 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. Core Data and Unit Testing haven't always been the best of friends. 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. Wait. Lightweight migrations carry little overhead. That's correct, versioning the data model is part of the solution. Download or clone the projectwe created in the previous tutorial and open it in Xcode. Each data model version has a unique identifier and Core Data stores this identifier in the persistent store to know what model was used to create it. Let me rephrase that. Versioning and migrations go … Crashes are no problem during development, but they are catastrophic in production. A mapping model defines how one version of the data model relates to another version. We need to tell Core Data how to migrate the persistent store for the data model. We do this when we add the persistent store to the persistent store coordinator in AppDelegate.swift. We then modified the data model by creating the User entity and defining several attributes and relationships. This indicates that Lists.xcdatamodel is the active data model version. We are closing in on the root of the problem. The Difference Between Light and Heavy Migrations There are two types of migrations for Core Data: light (also referred to as lightweight or inferred) migration and heavy (also referred to … - Selection from Core Data in Swift [Book] 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. If you don't invest time architecting the data model, chances are you run into problems that could have been avoided. With the data model selected, choose Add Model Version... from the Editor menu. In the development phase of a project we keep on adding new features in every versions of app. And you know what happened next. Finding out the cause of the crash is easy. 3) Lastly, it enforces any data validations in the final data model. Subscribe below and we’ll send you a weekly email summary of all new Code tutorials. Run the application in the simulator or on a physical device. Pre-Migration Preparation. Make sure that the project that you intend to migrate builds successfully in Swift 4 or Swift … 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. I strongly recommend that you choose a lightweight migration over a heavy migration whenever you can. Core Data tells us that the data model is not compatible with the data model we used to create the persistent store. Run the application in the simulator or on a physical device to make sure everything is set up correctly. Heavy migrations are a pain in the neck and you should try to avoid them if possible. Add a method to the subclass to convert the value. Modifying the names of entities, attributes, and relationship, however, is less trivial for Core Data. The first key, NSMigratePersistentStoresAutomaticallyOption, tells Core Data that we'd like it to attempt to migrate the persistent store for us. You can see the .xcdatamodeld file as a package for the versions of the data model, with each version represented by an .xcdatamodel file. pyrabbit / new_core_data_stack.swift. Core Data allows us to version the data model and this enables us to safely modify the data model. This makes it tempting to creat… Every form of persistence requires planning. You should also see an alert, informing the user about the problem. As a result, Core Data bails out and throws an error. You should see the two versions of the data model, Done.xcdatamodel and Done 2.xcdatamodel. In this article, we'll discuss two options to recover from such a situation, migrating the persistent store and creating a new persistent store that is compatible with the modified data model. Never miss out on learning about the next big thing. Name the version Lists 2 and base the data model version on Lists. In other words, the modified data model is no longer compatible with the persistent store, the SQLite database, it created earlier. However, if you want to make a change to the new model that’s not supported by lightweight migrations, you need to create a mapping model. Core Data is Apple’s object graph management and persistency framework. CoreData Swift: How to save and load data? So a while ago, I wrote on how I discovered the existence of this thing call Core Data migrations, by having my app crash when I added a new entity to my core data model.So the solution to that is actually not very hard, the only issue I had was that a majority of the solutions out there, deal with it in apps built with Objective-C and I needed a solution in Swift. This is exactly what we want. Run the application to see if we solved the incompatibility problem we ran into earlier. Also to address new changes we should not make any changes in existing data model. To solve the problem we're facing, we need to find a way to tell Core Data how one version of the data model relates to another version. Remember that the inverse relationship of the lists relationship is automatically set for us by Core Data. Lightweight migrations are ideal for expanding the data model, adding attributes and entities. Note that a green checkmark is added to Lists.xcdatamodel. When it comes to structured data persistence, Core Data is an obvious choice. To ensure Core Data can migrate the persistent store for us, it's important that you choose the previous version of the data model. Share ideas. Versioning and migrations go hand in hand. If we were to run the application, Core Data would continue to use the original data model version. For the previous release’s Migration Guide, see Migrating to Swift 4.2. If Core Data detects that the persistent store is incompatible with the data model, we first move the incompatible store to a safe location. Core Data Architecture Did that crash take you by surprise? Lightweight migrations require very little work from your part, the developer. Xcode will ask you to name the new data model version and, more importantly, on which version the new version should be based. Fortunately for us, a few clever engineers at Apple have created a solution to safely modify a data model without running into compatibility issues. How do you know if a data model change requires a lightweight or a heavyweight migration? In the implementation of the persistentStoreCoordinator property, we create the persistent store coordinator and add a persistent store to it by invoking addPersistentStoreWithType(_:configuration:URL:options:). However, because we changed the data model by adding an attribute to the, Core Data and Swift: More NSFetchedResultsController, Core Data and Swift: Subclassing NSManagedObject, Adobe Photoshop, Illustrator and InDesign. It is time to version the data model. Core Data and Swift: Migrations. I cannot stress enough how important this phase of a project is. When is a migration necessary? Basically the Core data migration happens in three steps: 1) Core data copies all objects from initial data store to the next. If adding the persistent store to the persistent store coordinator fails, the application invokes the fatalError() function, which causes the application to terminate immediately. 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. It's the only option available. Because we haven't run the application yet, we can still modify the new data model version without running into compatibility issues. Before we change this, though, we need to tell Core Data what it should do with the versioned data model. If you've followed the above steps, Core Data should automatically migrate the persistent store for you by inferring the mapping model based on the versioned data model. There are two types of migrations, lightweight and heavy migrations. The goal of this tutorial is to prevent that adding the persistent store to the persistent store coordinator fails. It is important that you understand the root cause of the problem. Hope you are doing good. We do this to make sure the user's data isn't lost. When a data model change is introduced, the developer uses EF Core tools to add a corresponding migration describing the updates necessary to keep the database schema in sync. To add support for migrations, we pass in a dictionary of options with two keys: What is a mapping model? We enter the world of heavy migrations if we make changes that Core Data cannot automatically infer for us by comparing versions of the data model. The cost of a lightweight migration is substantially lower than that of a heavy migration. In the previous articles of this series, we've encountered an annoying issue that we need to address. Core Data includes mechanisms for versioning your data model and migrating user data as your app evolves. A Heavy/Manual Migration A heavy migration is required when we go outside of the bounds of what a light migration can accomplish. Core Data inspects the versioned data model to understand how the persistent store relates to the data model. Core Data will then need a mapping model to understand how the versions of the data model relate to one another. In the section Model Version, set Current to Done 2. Avoiding abort is not that difficult, but it requires a few lines of code and informing the user about what went wrong in case something does go wrong. Avoid them whenever possible. In this example, we have only one choice. 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 Collaborate. An application that grows and gains features also gains new requirements. Let's revisit the to-do application we created in the previous article. For example, you could urge them to contact support and you can even implement a feature that lets them send you the corrupt store. To do that, we make use of two more helper methods, applicationIncompatibleStoresDirectory() and nameForIncompatibleStore(). Each entity version in each data model has a unique hash. With this change, we are ready to migrate the data model to the new version we created a few moments ago. Even if the data model is incompatible with the persistent store, you may be able to recover data form it. Tag: core-data-migration. Add a new attribute to the Item entity in Done 2.xcdatamodel and run the application one more time. Design templates, stock videos, photos & audio, and much more. This will take you to the data model in the Xcode project. This checkmark indicates what the current model version is, Done.xcdatamodel in this example. The implementation of applicationStoresDirectory(), a helper method, is straightforward as you can see below. 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.. Xcode 10.2 comes with a Swift Migrator tool that helps you migrate your project to Swift 5. That's what happens if you mess with the data model without telling Core Data about it. Migrations are an important topic if you plan to make extensive use of Core Data. Are you still running into a crash? If the persistent store coordinator is unable to add the existing persistent store at URLPersistentStore, we move the persistent store to a separate directory. With the data model selected, choose Add Model Version... from the Editor menu. The result of this action is that we can now see three data model files in the Project Navigator. The implementation of showAlertWithTitle(_:message:cancelButtonTitle:) is similar to the one we've seen in the AddToDoViewController. Type Date to the Item entity are closing in on the left the... Complex topic, we first and foremost leave the data model and name it.... Case of lightweight migrations, we have n't run the application one more time and how! Your luggage, pretending as if nothing happened is n't true for heavyweight migrations and that is what makes migrations... 'Ve changed the value of URLPersistentStore happy user and Done 2.xcdatamodel should now have a green has... 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