Core Data Programming Guide: Creating Managed Object Relationships
This also solves the common issue of incompatible Core Data models when the app is . 1-to-1 relationship to a User); You should see the issue there ; if I reload my Friends data (or my . Why not follow apples advice here. Core Data is an “object graph and persistence framework”, which is basically like a make sure bidirectional relationships are properly hooked up . of your UI should be reloaded: commands to update the UI come from a single This is more along the lines of “general good advice” than anything specific. I'm guessing because I'm not sure what you mean by "refresh". If it is faulted (not " realized" or not retrieved from the store), it will refresh when.
I am going to call it EatMoreVegetable. After saving the project at your favorite location, we should have a look at some of the files that Xcode generated for us.
Build a Simple Core Data Driven iOS App
A Core Data Template Having a look at the Navigator on the left, you should see one file that might be new to you: Core Data uses a scheme called a managed object model. A managed object model allows Core Data to map from records in a persistent store to managed objects that you use in your application.
You can think of entities as tables in a database. Their names are also the names of the classes subclasses of NSManagedObjectwhich are used to represent the entities in your application. The attributes you can add to an entity are also represented as properties in the NSManagedObject subclasses. For our Eat More Vegetable app, we only need one entity. So, click the round plus button at the bottom of the model editor. This will create a new entity which we can click once to select it, and once more to be able to rename it.
How Not To Fight NSFetchedResultsController and Core Data
Later, these attributes can be accessed in code, just like properties of class objects. Xcode automatically generates classes for entities added in the model editor. That is extremely convenient for us and we are going to have a look at our NSManagedObject subclass in a moment. Build the UI To get started, you may want to download these assets. These assets contain images that we are going to use as button images. The following short video is going to illustrate the process of creating the UI and how to connect it with the ViewController class.
When you are finished, continue reading on.
- Core Data Relationships
- How Not To Fight NSFetchedResultsController and Core Data
The Core Data Stack We have a data model, we also have a user interface. That means, the time of preparation is over and we can finally have a look at some code. Actually, the following code was already written for us when we activated Core Data capabilities for our project.
Reading and Updating Managed Objects With Core Data
It is located in the AppDelegate. So lets see what the code above does: This loads a persistent store and deals with error handling later on. We are not going into details on that. The most important part, if you plan to integrate Core Data into an already existing app, is the initialization of NSPersistentStore with the name of the used data model file without the extension xcdatamodel this does not necessarily have to be the App Name.
In my case this is simply EatMoreVegetable: A managed object context moc is your most important partner when interacting with Core Data. It is like an interface between your requests getting and adding information from and into the database. The moc sits at the top of the Core Data stack and could be simplified like this: To access the moc, you will need to use the viewContext property of the persistent container. How data is saved Xcode also generated a function at the very bottom of AppDelegate.
The function is pretty much self-explanatory. We need a place to store objects we get from Core Data and we also need a way to access our moc. These are the three properties that we are going to use for that. Add them right below the class definition in the ViewController. This an array of Food objects. Food is a NSManagedObject subclass that was automatically generated by Xcode, based on our data model and the food entity that we added.
Moc is going to be initialized with the moc. Our persistent container is defined in the AppDelegate class, which is why we need: To initialize the moc, we add the following line to the viewDidLoad function: Core Data fetches the missing data from the persistent store to fill in the gaps. Note that we only see a fault if the list record is fetched from the persistent store. A newly created managed object doesn't have any faults because it hasn't been fetched from a persistent store.
Take a closer look at the last print statement. Have you noticed that the items relationship is also a fault. The values of the name and createdAt attributes are resolved, but the items relationship isn't.
This is another Core Data optimization. Faulting is something you need to become familiar with if you plan to use Core Data. Most of the time, it's something you don't need to worry about. Updating Records How to Update an Attribute of a Managed Object The above output shows us that the list record we fetched from the persistent store doesn't have a name or creation date.
We can fix that by updating the list record and saving the changes. Run the application to update the list record. This is what you see in the console if the record doesn't have a name or creation date. It should look something like this: The documentation tells us that the return type of value forKey: This isn't ideal and it is something we fix later in this series.
Build a Simple Core Data Driven iOS App - Brian Advent
What's important is that we have updated the list record and pushed the updates to the persistent store. How to Update a Relationship of a Managed Object Working with relationships is similar to working with attributes. There are a few differences you need to be aware of. Setting or updating a to-one relationship is identical to setting or updating an attribute. Working with to-many and many-to-many relationships is different. The items relationship of the list record, for example, is a set NSSet of item records.
Most developers new to Core Data expect an array or an ordered set. To add or remove an item from a list, we need to update the set of NSManagedObject instances.
Let me show you how that works. In the example, we create an item record, set its attributes, set its list relationship, and add it to the items relationship of the list record. If you inspect the output in the console, you can see that the set of item records is unordered.
This makes adding and removing records from a to-many or many-to-many relationship a bit easier. In the example, we updated the list relationship of the item record and we also updated the items relationship of the list record.