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Workflow

The typical steps you take are defining your entities, specifying the attributes they have; specifying relationships between them; and adding business logic in the form of default values and value constraints. You may also define fetch templates for an entity. Although you can edit the model in the diagram view, it is more usual to do so in the browser, since it gives you more detail and greater flexibility. Creating a Managed Object Model Using Xcode provides a task-based approach to creating an entire model, from start to finish.

In this section:

Creating a Model
Custom Classes
Compiling a Data Model

Creating a Model

If you create a Core Data–based project, a data model is automatically created for you and added to the project. If you need to create a new model, from the File menu choose New File and add a file of type Data Model (from the Design list). In the pane that appears (see Figure 12-14), give the file a suitable name and ensure that the file is added to your application target.

Click Next, and in the following pane select any groups or files that you want to be parsed for inclusion in the model (if any), then click Finish.

Custom Classes

For each entity in the model, you specify a class that will be used to represent it in your application. By default the class is set to NSManagedObject, which is able to represent any entity. Typically, at the begnining of a project, you just use NSManagedObject for all your entities. Later, as your project matures, you define custom subclasses of NSManagedObject to provide custom functionality.

If you create a custom subclass of NSManagedObject to represent an entity, you typically implement custom accessor methods for the class’s properties. This is generally tedious, repetitive work, so the data modeling tool provides menu items to automatically generate declarations and implementations for these methods and put them on the Clipboard so you can paste them into the appropriate source file.

Compiling a Data Model

A data model is a deployment resource. A data model must not only be a project file, it must be associated with the target that uses it . In addition to details of the entities and properties in the model, the model contains information about the diagram, its layout, colors of elements, and so on. This latter information is not needed at runtime. The model file is compiled to remove the extraneous information and make runtime loading of the resource as efficient as possible.

The model compiler is momc in /Library/Application Support/Apple/Developer Tools/Plug-ins/XDCoreDataModel.xdplugin/Contents/Resources/. If you want to use it in your own build scripts, its usage is: momc source destination, where source is the path of the Core Data model to compile, and destination is the path of the output mom file.

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