Data Model, Entities, and Attributes
The data model is a key component of the Core Data stack and an integral part of every Core Data application. In this tutorial, we explore the data model and learn about entities and attributes.
The best way to learn is by doing. Open Xcode and create a new project, choosing the Single View Application template. Name the project Data Model and tell Xcode to populate the project with boilerplate code for Core Data by checking the Use Core Data checkbox.
Take a quick look at AppDelegate.swift to refresh your memory. The application delegate creates an instance of the NSPersistentContainer class, which is responsible for setting up and managing the Core Data stack.
When the application sets up the Core Data stack, the managed object model loads the data model from the application bundle. The file that’s loaded from the application bundle is named Data_Model and has a .momd extension. The extension stands for managed object model document.
If you open the Project Navigator on the left, you should see a file named Data_Model. The file’s extension, .xcdatamodeld, doesn’t match, though. And yet if you run the application, the Core Data stack is set up without issues. How is that possible?
When the application is built, the data model file you see in the Project Navigator is compiled into a .momd file. Why is that necessary? Why does the data model need to be compiled?
The .xcdatamodeld file is the file we edit in Xcode during development. We use it for defining the application’s data model. It shows us a visual representation of the data model as we will see in a moment. Much of the information in the .xcdatamodeld file is not needed for Core Data to do its work. At compile time, Xcode collects the data it needs from the .xcdatamodeld file and creates a .momd file. It is the .momd file that is included in the application bundle. The resulting .momd file is much smaller, containing only what is absolutely essential for Core Data to infer the data model.
Read this article on Cocoacasts.