A description of a property of a Core Data entity.
SDKs
- iOS 3.0+
- macOS 10.4+
- Mac Catalyst 13.0+
- tvOS 9.0+
- watchOS 2.0+
Framework
- Core Data
Declaration
@interface NSPropertyDescription : NSObject
Overview
A property describes a single value within an object managed by the Core Data Framework. There are different types of property, each represented by a subclass which encapsulates the specific property behavior—see NSAttribute, NSRelationship, and NSFetched.
Note that a property name cannot be the same as any no-parameter method name of NSObject or NSManaged. For example, you cannot give a property the name "description". There are hundreds of methods on NSObject which may conflict with property names—and this list can grow without warning from frameworks or other libraries. You should avoid very general words (like “font”, and “color”) and words or phrases which overlap with Cocoa paradigms (such as “isEditing” and “objectSpecifier”).
Properties—relationships as well as attributes—may be transient. A managed object context knows about transient properties and tracks changes made to them. Transient properties are ignored by the persistent store, and not just during saves: you cannot fetch using a predicate based on transients (although you can use transient properties to filter in memory yourself).
Editing Property Descriptions
Property descriptions are editable until they are used by an object graph manager (such as a persistent store coordinator). This allows you to create or modify them dynamically. However, once a description is used (when the managed object model to which it belongs is associated with a persistent store coordinator), it must not (indeed cannot) be changed. This is enforced at runtime: any attempt to mutate a model or any of its sub-objects after the model is associated with a persistent store coordinator causes an exception to be thrown. If you need to modify a model that is in use, create a copy, modify the copy, and then discard the objects with the old model.