Simple core data relationship example

Core Data on iOS 10, a brief overview with an Example

simple core data relationship example

Create a new project and, for convenience, check Use Core Data during the setup of the project. To keep it simple, I will create an account record and a user record in the Take a look at the updated example below. Core Data is an object graph and persistence framework provided by each entity there but for us Attributes and Relationships will be most important. with predicate for the entity (User in our example); Fetch record and Set. Basics of CoreData framework, which is a tool for iOS, macOS, tvOS A sample project that we will be creating will be build with Xcode 9 and Swift 4. Than you have to get back to User entity and set relationship with Car.

For example add a new attribute to a entity, and re-run the application with previous stored data. In this situation, you have two options. You can prepare a migration, if you already have your application on the App Store. And finally the Managed Object Context, which is the one that your source code will be interacting with the most, is responsible for managing objects created and returned using Core Data.

simple core data relationship example

For object graph changes that affect multiple objects, it is much faster if they are already in memory, rather than searching for them again in the database. You can modify those objects however you like, and unless you actually save those changes, the persistent store remains unaltered.

And by tracking changes, the context is able to provide undo and redo support for you. After you create your project, you will find the. Go ahead and select that file to open the Xcode Data Model editor as shown below. Go ahead and click that option to create a new entity. You can change the name like you would change a file name on Finder, but you can also use the inspector to perform this change. After you press enter, the Class name, below the section Entity, will also be updated with the desired name.

In this case, the editor will be expecting you to change the name and select a valid type. After you create this new attribute it should look like the picture below.

That is one of the responsabilities of Core Data and it is the framework that will create the logic for your entity internally. After this we want to create a relationship between these two entities, where a task can have multiple notes. You will notice that a warning appears. This is not strictly required, but setting the inverse relationship will help Core Data ensure the consistency of the object graph when a change is made.

simple core data relationship example

Initializing the Persistence Coordinator In iOS 10, you only need to specify the container name with the same name as your. Designing a simple User Interface Since the main goal of this article is not related to how we design user interfaces, I will give you a quick setup to support this article. To start, go ahead and select the Main. It should look like the picture below. Next, we need to indicate to the Table View which object will be responsible to return objects for it.

  • Adding Core Data entity relationships: lightweight vs heavyweight migration
  • Getting Started with Core Data Tutorial
  • Core Data Relationships

Going back to code, add the following instance and method into the ViewController. And with this we have a basic user interface to support this use case for Code Data. I will not extend all user interfaces to fulfill the entire model layer, but you have the idea how to propagate to additional view controllers.

Then add to your ViewController. To fetch objects from the database the best aproach, and the solution Apple wants us to use, is the NSFetchedResultsController.

This will take you back to the familiar app grid on the home screen: From the home screen, tap the HitList icon to bring the app back to the foreground. The names are still on the screen. When you tap the Home button, the app currently in the foreground goes to the background. When this happens, the operating system flash-freezes everything currently in memory, including the strings in the names array.

Apple introduced these advances in multitasking back in iOS 4.

simple core data relationship example

They create a seamless experience for iOS users but add a wrinkle to the definition of persistence for iOS developers. Are the names really persisted?

Core Data Relationships – Hacker Noon

If you had completely killed the app in the fast app switcher or turned off your phone, those names would be gone. You can verify this as well. With the app in the foreground, enter the fast app switcher.

From here, flick the HitList app snapshot upwards to terminate the app. After you remove the app from the app switcher, there should be no trace of HitList in living memory no pun intended. Verify the names are gone by returning to the home screen and tapping on the HitList icon to trigger a fresh launch. All that matters is the names are still there when the app comes back!

So the real test of persistence is whether your data is still there after a fresh app launch. Your goal for the HitList app is simple: The first step is to create a managed object model, which describes the way Core Data represents data on disk. For a comprehensive list of Core Data classes, check out the Core Data framework reference in the documentation browser.

As you can see, Xcode has a powerful Data Model editor: The Data Model editor has a lot of features you can explore later. Click on Add Entity on the lower-left to create a new entity. Double-click the new entity and change its name to Person, like so: You may be wondering why the model editor uses the term Entity.

An entity is a class definition in Core Data. The classic example is an Employee or a Company. In a relational database, an entity corresponds to a table. An attribute is a piece of information attached to a particular entity. In a database, an attribute corresponds to a particular field in a table.

One-to-Many and Many-to-Many Core Data Relationships

A relationship is a link between multiple entities. In Core Data, relationships between two entities are called to-one relationships, while those between one and many entities are called to-many relationships. For example, a Manager can have a to-many relationship with a set of employees, whereas an individual Employee will usually have a to-one relationship with his manager. Likewise, attributes and relationships sound a lot like properties. You can think of a Core Data entity as a class definition and the managed object as an instance of that class.

Now you know what an attribute is, you can add an attribute to Person object created earlier. In Core Data, an attribute can be of one of several data types. Next, replace the names property definition with the following: It now holds instances of NSManagedObject rather than simple strings. NSManagedObject represents a single object stored in Core Data; you must use it to create, edit, save and delete from your Core Data persistent store. It can take the form of any entity in your Data Model, appropriating whatever attributes and relationships you defined.

Instead of matching cells with the corresponding string in the model array, you now match cells with the corresponding NSManagedObject. Note how you grab the name attribute from the NSManagedObject. String Why do you have to do this? Xcode complains because save name: Think of saving a new managed object to Core Data as a two-step process: Remember, this only happens if you check the Use Core Data checkbox at the beginning.

This default managed object context lives as a property of the NSPersistentContainer in the application delegate. To access it, you first get a reference to the app delegate.

simple core data relationship example

You create a new managed object and insert it into the managed object context. You may be wondering what an NSEntityDescription is all about. Recall earlier, NSManagedObject was called a shape-shifter class because it can represent any entity. An entity description is the piece linking the entity definition from your Data Model with an instance of NSManagedObject at runtime.

With an NSManagedObject in hand, you set the name attribute using key-value coding. You must spell the KVC key name in this case exactly as it appears in your Data Model, otherwise, your app will crash at runtime. You commit your changes to person and save to disk by calling save on the managed object context. Note save can throw an error, which is why you call it using the try keyword within a do-catch block.

Finally, insert the new managed object into the people array so it shows up when the table view reloads. Some of the code here, such as getting the managed object context and entity, could be done just once in your own init or viewDidLoad then reused later.

simple core data relationship example

Build and run the app, and add a few names to the table view: If the names are actually stored in Core Data, the HitList app should pass the persistence test. With the app in the foreground, go to the fast app switcher and then terminate it. From Springboard, tap the HitList app to trigger a fresh launch. The table view is empty: You saved to Core Data, but after a fresh app launch, the people array is empty! Fetching from Core Data To get data from your persistent store into the managed object context, you have to fetch it.

Before you can do anything with Core Data, you need a managed object context. Fetching is no different! Like before, you pull up the application delegate and grab a reference to its persistent container to get your hands on its NSManagedObjectContext. Fetch requests are both powerful and flexible.