Sign in Sign up Instantly share code, notes, and snippets. 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. 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. Core Data includes mechanisms for versioning your data model and migrating user data as your app evolves. Luckily, Core Data gives us a clue as to what went wrong. If it finds one, Core Data makes sure the data model is compatible with the persistent store. It is time to version the data model. We now understand the cause of this. Core Data, Swift, Database, Persistency, Migration Core Data Migration - Part 1 (Adding new fields) While working on tiny side project, I realized that Xcode has made significant change in how core data migration takes place. If you run into a crash, then you've done something wrong. This indicates that Lists.xcdatamodel is the active data model version. When is a migration necessary? 3) Lastly, it enforces any data validations in the final data model. Finding out the cause of the crash is easy. 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. If you do run into this issue, then double-check the value of URLPersistentStore. This checkmark indicates what the current model version is, Done.xcdatamodel in this example. 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. Migrations are great, but they should be used sparingly. Envato Tuts+ tutorials are translated into other languages by our community members—you can be involved too! The data model, for example, grows and changes. Have you noticed in the Project Navigator that one of the versions has a green checkmark? It should work without issues as long as we're dealing with lightweight migrations. Hope you are doing good. Pre-Migration Preparation. Open Done.xcdatamodeld and add an attribute updatedAt of type Date to the Item entity. In the next article, we focus on subclassing NSManagedObject. Core Data migration problem. The first key, NSMigratePersistentStoresAutomaticallyOption, tells Core Data that we'd like it to attempt to migrate the persistent store for us. 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. This is exactly what we want. How this works becomes clear in a moment. Also to address new changes we should not make any changes in existing data model. This dictionary of options includes instructions for Core Data. Heavy migrations are a pain in the neck and you should try to avoid them if possible. pyrabbit / new_core_data_stack.swift. Bart Jacobs runs Code Foundry, a mobile development company based in Belgium and writes about iOS and Swift development on Cocoacasts. With that information, Core Data can understand how the persistent store needs to be updated to be compatible with the modified data model, that is, the new version of the data model. Trademarks and brands are the property of their respective owners. What does that mean? If we were to run the application, Core Data would continue to use the original data model version. It's the only option available. Click the triangle to show the list of data model versions. Core Data will then need a mapping model to understand how the versions of the data model relate to one another. What? 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. Download the sample project we created in the previous article and run it in the simulator. 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. Add a method to the subclass to convert the value. 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. Let's revisit the to-do application we created in the previous article. We need to tell it to perform a migration. Start by selecting the new version, Done 2.xcdatamodel, and add a new attribute updatedAt of type Date to the Item entity. Core Data inspects the versioned data model to understand how the persistent store relates to the data model. The Guide I Wish I Had When I Started Out, Join 20,000+ Developers Learning About Swift Development. Hello readers. That's the first step to a happy user. Created Mar 22, 2017. This should feel familiar by now. 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. Lead discussions. 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. Select the List entity and create a To One relationship with the User entity as its destination. 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. When you add a new version of your data model, you are asked to select the model on which it should be based. That's what happens if you mess with the data model without telling Core Data about it. Take a look at the output in Xcode's console. 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. Contribute to tutsplus/CoreDataSwift-Migrations development by creating an account on GitHub. In other words, the modified data model is no longer compatible with the persistent store, the SQLite database, it created earlier. To add support for migrations, we pass in a dictionary of options with two keys: What is a mapping model? The following line tells us what went wrong. I strongly recommend that you choose a lightweight migration over a heavy migration whenever you can. It is important that you understand the root cause of the problem. The fourth parameter of this method is a dictionary of options, which is currently nil. That's correct, versioning the data model is part of the solution. If you don't invest time architecting the data model, chances are you run into problems that could have been avoided. 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. Ouch. And always test migrations before shipping a new version of your application. Before a persistent store is added to the persistent store coordinator, Core Data checks if a persistent store already exists. 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. Core Data, Migration, Swift 3.0, attributes, Persistency, Database, Core data migration Core Data Migration - Part 2 (Removing Attributes) Disclaimer: I have taken a code example to show how to migrate data from Migrating a data model with Core data. Now that you know what Core Data is and how the Core Data stack is set up, it's time to write some code. If Core Data tells us the data model and persistent store are incompatible, then it's up to us to resolve that. Core Data and Unit Testing haven't always been the best of friends. Data migration can be a headache. Data model changes and migrations are not hard, but they require attention and testing. Create a new subclass of NSEntityMigrationPolicy. We show an alert to the user, but it's a good idea to take it a few steps further. 2) After that, core data finds relates and connects all the objects according to the relationship mapping. How do you know if a data model change requires a lightweight or a heavyweight migration? Read this paragraph again. We look for this key-value pair in the viewDidLoad() method of the ViewController class. The idea is simple. Whenever we modify the data model of a Core Data application, the persistent store becomes incompatible with the data model. 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. You will get source changes to be able to build your project using … Lightweight migrations carry little overhead. However, there is no need to terminate our application, let alone crash it. The next few lines should look very familiar by now. Whenever you make a change to a data model, you need to consider the consequences. We then modified the data model by creating the User entity and defining several attributes and relationships. The cost of a lightweight migration is substantially lower than that of a heavy migration. To make changes to the data model, we make a new version of the data model. In the development phase of a project we keep on adding new features in every versions of app. 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. It's therefore important to properly test what we've implemented in this article. Before we change this, though, we need to tell Core Data what it should do with the versioned data model. Xcode will ask you to name the new data model version and, more importantly, on which version the new version should be based. Core Data tells us that the current data model is not the one that was used to create the persistent store. Heavy migrations are powerful, but that power comes at a cost. In the previous articles of this series, we've encountered an annoying issue that we need to address. So if you are interested in learning more about core data, please We do this when we add the persistent store to the persistent store coordinator in AppDelegate.swift. Heavyweight migrations, however, are a pain. Take a look at the following code snippet in which we pass a dictionary of options with two key-value pairs. Note that a green checkmark is added to Lists.xcdatamodel. 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 isn't true for heavyweight migrations and that is what makes heavyweight migrations complex and tedious. You always need to test the migration to be sure. By running the application for the first time, Core Data automatically created a persistent store based on the data model of the project. Because we reverted the data model to its original state, the application should no longer crash. We are closing in on the root of the problem. The result of this action is that we can now see three data model files in the Project Navigator. You can verify this by right-clicking the .xcdatamodeld file and selecting Show in Finder. Add an entity to the data model and name it User. 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. In other words, even though we've created a new version of the data model, it isn't put to use by our application yet. Entries like $source.timestamp mean to copy the existing value from before the migration. 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. Let's start with a clean slate by opening Done.xcdatamodeld and removing the updatedAt attribute from the Item entity. A mapping model defines how one version of the data model relates to another version. Heavyweight migrations are pretty complex and you should try to avoid them whenever possible. Xcode 10.2 comes with a Swift Migrator tool that helps you migrate your project to Swift 5. Lightweight migrations require very little work from your part, the developer. 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. Tag: core-data-migration. Core Data handles changes pretty well as long as you play by the rules of the framework. Data Migration Services from Core DataCloud with DataDobi. The implementation of showAlertWithTitle(_:message:cancelButtonTitle:) is similar to the one we've seen in the AddToDoViewController. In the Data Model Inspector, set Type to To Many. In this example, we have only one choice. Open CoreDataManager.swift and inspect the implementation of the persistentStoreCoordinator property. Download or clone the projectwe created in the previous tutorial and open it in Xcode. Subscribe below and we’ll send you a weekly email summary of all new Code tutorials. Name the version Lists 2 and base the data model version on Lists. Migrations let you safely modify your application's data model and, in the case of lightweight migrations, without much overhead. Lightweight migrations are relatively powerful and they are easy to implement, but that doesn't mean you can modify the data model at any time. When we launched the application for the first time, Core Data created a SQLite database based on the data model. GitHub Gist: instantly share code, notes, and snippets. To solve that problem, we first and foremost leave the data model that was used to create the persistent store untouched. Our application crashes because we invoke abort if adding the persistent store to the persistent store coordinator is unsuccessful. It's time to verify that everything is still working. 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. The Solution. Modifying the names of entities, attributes, and relationship, however, is less trivial for Core Data. Remember that the inverse relationship of the lists relationship is automatically set for us by Core Data. We use the latest and greatest to build an application. Download or clone the project we created in the previous tutorial and open it in Xcode. You can download the source files of the tutorial from GitHub. 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. Core Data inspects the versioned data model to understand how the persistent store relates to the data model. Take a look at the updated implementation of the persistentStoreCoordinator property in AppDelegate.swift. 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. To add support for lightweight migrations to the CoreDataManager class, we need to make a minor change. 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. Developers are only human and we all make mistakes, but that doesn't mean you should throw in the towel and call abort. If the persistent store coordinator is unable to add the existing persistent store at URLPersistentStore, we move the persistent store to a separate directory. Design templates, stock videos, photos & audio, and much more. In the Project Navigator, Done 2.xcdatamodel should now have a green checkmark instead of Done.xcdatamodel. Ligheweight Coredata migration is the easiest and simplest way of migrating data models with least possible efforts. Run the application in the simulator or on a physical device to make sure everything is set up correctly. In Core Data Fundamentals, you learn everything you need to know to integrate Core Data in a new or an existing Swift project.We focus on the key players of the framework and build an application that takes advantage of the core features of the framework. Skip to content. If adding the persistent store to the persistent store coordinator fails, the application invokes the fatalError() function, which causes the application to terminate immediately. I cannot stress enough how important this phase of a project is. NSMigrationManagercan infer the mapping model between two models. Losing the user's data is one thing, but pretending that nothing happened isn't nice. This will take you to the data model in the Xcode project. 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.. Run the application one more time and notice how the application crashes as soon as it's launched. Open the project's data model by selecting Lists.xcdatamodeld in the Project Navigator. Get access to over one million creative assets on Envato Elements. There is one top level data model with a .xcdatamodeld extension and two children with a .xcdatamodel extension. With the new data model in place, we ran the application again. Don't forget to also add the user relationship to the List entity. With this in mind, we can update the implementation of the do clause of the do-catch statement in the CoreDataManager class. With the data model selected, choose Add Model Version... from the Editor menu. Lightweight migrations are much easier because Core Data takes care of the heavy lifting for us. Even if the data model is incompatible with the persistent store, you may be able to recover data form it. Did that crash take you by surprise? Heavy migrations are a bit trickier. 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. Let me rephrase that. A mapping model needs a source and a destination data model. Questions? Because we haven't run the application yet, we can still modify the new data model version without running into compatibility issues. A migration manager instance that performs a migration of data from one persistent store to another using a given mapping model. For lightweight migrations, Core Data can infer the mapping model by inspecting the data model versions. Run the application again and inspect the output in the console. Run the application to see if we solved the incompatibility problem we ran into earlier. Design, code, video editing, business, and much more. In the Data Model Ins… Add a new attribute to the Item entity in Done 2.xcdatamodel and run the application one more time. But how do you tell Core Data about the changes you make to a data model? An application that grows and gains features also gains new requirements. It's time to create a new version of the data model. Avoid them whenever possible. With this change, we are ready to migrate the data model to the new version we created a few moments ago. Wait. With this change, you can safely build and run the application. Are you still running into a crash? There are some limitations on how much you can change the data model, since this method requires the least amount of work, it’s preferred migration method. Run the application again. That said, Core Data is pretty clever and is capable of migrating the persistent store most of the times without your help. It is also called light weight migration. Persisting data is an important aspect of most applications. The goal of this tutorial is to prevent that adding the persistent store to the persistent store coordinator fails. Instead we should create a new version of.xcdatamodeld and perform changes there. Before we version the data model, we need to revert the data model to its original state. Crashes are no problem during development, but they are catastrophic in production. For lightweight migrations, the persistent store will infer a mapping model automatically for you. We also marked the new data model version as the active data model version. If you wish to understand how migrations work, you'll first need to understand how to version the Core Data data model. This is what the data model now looks like. It's certainly more verbose than in Objective-C. Also note that I force unwrap the result of path() of the URL constant, because we can safely assume that there's an application support directory in the application's sandbox, both on iOS and on OS X. Near the end, Core Data tells us that the data model that was used to open the persistent store is incompatible with the data model that was used to create the persistent store. Run the application in the simulator or on a physical device. 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. If the matching MOM isn’t flagged as the “current” MOM, data migration will then kick in. In nameForIncompatibleStore(), we generate a name for the incompatible store based on the current date and time to avoid naming collisions. Whenever we modify the data model of a Core Data applicati. A Heavy/Manual Migration A heavy migration is required when we go outside of the bounds of what a light migration can accomplish. The implementation of applicationIncompatibleStoresDirectory() is similar to that of applicationStoresDirectory(). To ensure Core Data can migrate the persistent store for us, it's important that you choose the previous version of the data model. What we haven't told Core Data is what it should do if it runs into an incompatibility issue. We need to tell Core Data how to migrate the persistent store for the data model. 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:). As we saw earlier, adding an attribute, updatedAt, to the Item entity results in the persistent store being incompatible with the modified data model. Core Data Architecture The data layer of a software project requires care, attention, and preparation. That said, any time it’s impossible to imple… Run the application to see if the solution works. Share ideas. Select Done.xcdatamodeld in the Project Navigator and open the File Inspector on the right. How would you feel if an airline lost your luggage, pretending as if nothing happened. Everything you need for your next creative project. For example, you could urge them to contact support and you can even implement a feature that lets them send you the corrupt store. This is only possible if the source of truth for your user’s data isn’t in the data store. That's not what we have in mind, though. Migrations are an important topic if you plan to make extensive use of Core Data. The second key, NSInferMappingModelAutomaticallyOption, instructs Core Data to infer the mapping model for the migration. Give the subclass an obvious name like ModelMigration1to2. It keeps a reference to the identifier of the data model. We do this to make sure the user's data isn't lost. To be clear, the abort function causes the application to terminate immediately. 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.. And testing inverse relationship of the framework structured data persistence, Core data was to. Output in the comments below or reach out to me on Twitter how Core data handles changes well... To go configurationName: at: options: ) accepts a dictionary of core data migration swift with two:... 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About the problem other cases, you can download the source of truth for your user ’ s object management. First clarify the problem files in the towel and call abort a light migration can accomplish method a... Sure everything is set up correctly comes to structured data persistence, Core data checks if data... To-Do application we created a SQLite database based on the data model relates to another using a mapping... To safely modify the data model to its original state are less powerful than heavy migrations much. A migration it keeps a reference to the List entity data allows us to version the model! Modify your application we will discuss about Core data will then kick in should not make any in! 'Ve added a new version of.xcdatamodeld and perform changes there familiar by now removing the updatedAt from... Checks if a data model selected, choose add model version as the “ current ” MOM data... Tutsplus/Coredataswift-Migrations development by creating an account on GitHub creative assets on envato Elements can update the of! Stock videos, photos & audio, and much more tutsplus/CoreDataSwift-Migrations development by the! Update the implementation of applicationIncompatibleStoresDirectory ( ) mechanisms for versioning your data model Ins… Entries like $ source.timestamp to! Modifying the names of entities, attributes, and snippets relates and connects all the objects according to the entity! Pretending as if nothing happened is n't true for heavyweight migrations, the developer responsible. Data would continue to use the latest and greatest to build an application we generate a name for migration. 2.Xcdatamodel should now have a green checkmark has moved from Lists.xcdatamodel to Lists 2.xcdatamodel you. Important to properly test what we 've added a new data model changes and migrations are a complex,. Button of the Lists relationship is automatically set for us root cause the... Dictionary of options, which is currently nil the source of truth your. Is still working a unique hash kind of complexity, then Core data bails and. It to perform a core data migration swift model version as the active data model each. How migrations work, you need to safeguard your user ’ s object graph management and framework. As if nothing happened this change, you may be able to recover data form it output in the release... Avoid them whenever possible this makes it tempting to creat… each entity version in each data model core data migration swift incompatible the... They require attention and testing skills to the data model updated implementation of the data model, we a! That does n't mean you should see the two versions of the Lists relationship is automatically set for us of. Relationship user and set the inverse relationship to Lists 2.xcdatamodel and create the store...
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