Class

NSManagedObject

A base class that implements the behavior required of a Core Data model object.

Declaration

@interface NSManagedObject : NSObject

Overview

A managed object is associated with an entity description (NSEntityDescription) that provides metadata about the object, including the name of the entity that the object represents and the names of its attributes and relationships. A managed object is also associated with a managed object context that tracks changes to the object graph.

You can’t use instances of direct subclasses of NSObject (or any other class that doesn’t inherit from NSManagedObject) with a managed object context. You may create custom subclasses of NSManagedObject, although this isn’t always required. If no custom logic is needed, you can create a complete Object graph with NSManagedObject instances.

If you instantiate a managed object directly, you must call the designated initializer initWithEntity:insertIntoManagedObjectContext:.

Data Storage

In some respects, an NSManagedObject acts like a dictionary—it’s a generic container object that provides efficient storage for the properties defined by its associated NSEntityDescription instance. NSManagedObject supports a range of common types for attribute values, including string, date, and number (see NSAttributeDescription for full details). Therefore, typically you don’t need to define instance variables in subclasses. Sometimes, however, you want to use types that aren’t supported directly, such as colors and C structures. For example, in a graphics application you might want to define a Rectangle entity that has color and bounds attributes that are an instance of NSColor and an NSRect struct, respectively. For some types you can use a transformable attribute, for others this may require you to create a subclass of NSManagedObject.

Faulting

Managed objects typically represent data held in a persistent store. In some situations a managed object may be a fault—an object whose property values haven’t yet been loaded from the external data store. When you access persistent property values, the fault “fires” and the data is retrieved from the store automatically. This can be a comparatively expensive process (potentially requiring a round trip to the persistent store), and you may wish to avoid unnecessarily firing a fault. See Faulting and Uniquing for more details on faults.

You can safely invoke the following methods and properties on a fault without causing it to fire: isEqual:, hash, superclass, class, self, isProxy, isKindOfClass:, isMemberOfClass:, conformsToProtocol:, respondsToSelector:, description, managedObjectContext, entity, objectID, inserted, updated, deleted, faultingState, and fault. Because isEqual and hash don’t cause a fault to fire, managed objects can typically be placed in collections without firing a fault. Note, however, that invoking key-value coding methods on the collection object might in turn result in an invocation of valueForKey: on a managed object, which would fire the fault.

Although the description property doesn’t cause a fault to fire, if you implement a custom description that accesses the object’s persistent properties, this does cause a fault to fire. You are strongly discouraged from overriding description in this way.

Subclassing Notes

In combination with the entity description in the managed object model, NSManagedObject provides a rich set of default behaviors including support for arbitrary properties and value validation. If you decide to subclass NSManagedObject to implement custom features, make sure you don’t disrupt Core Data’s behavior.

Methods and Properties You Must Not Override

NSManagedObject itself customizes many features of NSObject so that managed objects can be properly integrated into the Core Data infrastructure. Core Data relies on the NSManagedObject implementation of the following methods and properties, which you therefore absolutely must not override: primitiveValueForKey:, setPrimitiveValue:forKey:, isEqual:, hash, superclass, class, self, isProxy, isKindOfClass:, isMemberOfClass:, conformsToProtocol:, respondsToSelector:, managedObjectContext, entity, objectID, inserted, updated, deleted, and fault, alloc, allocWithZone:, new, instancesRespondToSelector:, instanceMethodForSelector:, methodForSelector:, methodSignatureForSelector:, instanceMethodSignatureForSelector:, or isSubclassOfClass:.

Methods and Properties You Shouldn't Override

As with any class, you are strongly discouraged from overriding the key-value observing methods such as willChangeValueForKey: and didChangeValueForKey:withSetMutation:usingObjects:. Avoid overriding description—if this method fires a fault during a debugging operation, the results may be unpredictable. Also avoid overriding initWithEntity:insertIntoManagedObjectContext:, or dealloc. Changing values in the initWithEntity:insertIntoManagedObjectContext: method won't be noticed by the context, and if you aren't careful, those changes may not be saved. Perform most initialization customization in one of the awake… methods. If you do override initWithEntity:insertIntoManagedObjectContext:, make sure you adhere to the requirements set out in the method description. See initWithEntity:insertIntoManagedObjectContext:.

Don’t override dealloc because didTurnIntoFault is usually a better time to clear values—a managed object may not be reclaimed for some time after it has been turned into a fault. Core Data doesn’t guarantee that dealloc will be called in all scenarios (such as when the application quits). Therefore, don’t include required side effects (like saving or changes to the file system, user preferences, and so on) in these methods.

In summary, for initWithEntity:insertIntoManagedObjectContext: and dealloc, Core Data reserves exclusive control over the life cycle of the managed object (that is, raw memory management). This is so that the framework can provide features such as uniquing and by consequence, relationship maintenance, as well as much better performance than would be possible otherwise.

Additional Override Considerations

The following methods are intended to be fine grained and aren’t suitable for large-scale operations. Don’t fetch or save in these methods. In particular, they shouldn’t have side effects on the managed object context.

  • initWithEntity:insertIntoManagedObjectContext:

  • didTurnIntoFault

  • willTurnIntoFault

  • dealloc

In addition, if you plan to override awakeFromInsert, awakeFromFetch, and validation methods, first invoke super.method(), the superclass’s implementation. Don’t modify relationships in awakeFromFetch—see the method description for details.

Custom Accessor Methods

Typically, you don’t need to write custom accessor methods for properties that are defined in the entity of a managed object’s corresponding managed object model. If you need to do so, follow the implementation patterns described in Managed Object Accessor Methods in Core Data Programming Guide.

Core Data automatically generates accessor methods (and primitive accessor methods) for you. For attributes and to-one relationships, Core Data generates the standard get and set accessor methods; for to-many relationships, Core Data generates the indexed accessor methods as described in Achieving Basic Key-Value Coding Compliance in Key-Value Coding Programming Guide. You do however need to declare the accessor methods or use Objective-C properties to suppress compiler warnings. For a full discussion, see Managed Object Accessor Methods in Core Data Programming Guide.

Custom Instance Variables

By default, NSManagedObject stores its properties in an internal structure as objects, and in general Core Data is more efficient working with storage under its own control rather than by using custom instance variables.

NSManagedObject provides support for a range of common types for attribute values, including string, date, and number (see NSAttributeDescription for full details). If you want to use types that aren’t supported directly, like colors and C structures, you can either use transformable attributes or create a subclass of NSManagedObject.

Sometimes it’s convenient to represent variables as scalars—in drawing applications, for example, where variables represent dimensions and x and y coordinates and are frequently used in calculations. To represent attributes as scalars, you declare instance variables as you do in any other class. You also need to implement suitable accessor methods as described in Managed Object Accessor Methods.

If you define custom instance variables for example to store derived attributes or other transient properties, clean up these variables in didTurnIntoFault rather than dealloc.

Validation Methods

NSManagedObject provides consistent hooks for validating property and inter-property values. You typically shouldn’t override validateValue:forKey:error:. Instead implement methods of the form validate<Key>:error:, as defined by the NSKeyValueCoding protocol. If you want to validate inter-property values, you can override validateForUpdate: and/or related validation methods.

Don’t call validateValue:forKey:error: within custom property validation methods—if you do, you create an infinite loop when validateValue:forKey:error: is invoked at runtime. If you do implement custom validation methods, don’t call them directly. Instead, call validateValue:forKey:error: with the appropriate key. This ensures that any constraints defined in the managed object model are applied.

If you implement custom inter-property validation methods like validateForUpdate:, call the superclass’s implementation first. This ensures that individual property validation methods are also invoked. If there are multiple validation failures in one operation, collect them in an array and add the array—using the key NSDetailedErrorsKey—to the userInfo dictionary in the NSError object you return. For an example, see Managed Object Validation.

Topics

Initializing a Managed Object

- initWithEntity:insertIntoManagedObjectContext:

Initializes a managed object from an entity description and inserts it into the specified managed object context.

- initWithContext:

Initializes a managed object subclass and inserts it into the specified managed object context.

Getting a Managed Object’s Identity

entity

The entity description of the managed object.

objectID

The object ID of the managed object.

+ entity

Returns the entity description that is associated with this subclass.

Getting State Information

managedObjectContext

The managed object context with which the managed object is registered.

hasChanges

A Boolean value that indicates whether the managed object has been inserted, has been deleted, or has unsaved changes.

inserted

A Boolean value that indicates whether the managed object has been inserted in a managed object context.

updated

A Boolean value that indicates whether the managed object has unsaved changes.

deleted

A Boolean value that indicates whether the managed object will be deleted during the next save.

fault

A Boolean value that indicates whether the managed object is a fault.

faultingState

The faulting state of the managed object.

- hasFaultForRelationshipNamed:

Returns a Boolean value that indicates whether the relationship for a given key is a fault.

hasPersistentChangedValues

A Boolean value that indicates whether the managed object has persistent changes.

Managing Change Events

contextShouldIgnoreUnmodeledPropertyChanges

A Boolean value that indicates whether instances of the class should be marked as having changes if an unmodeled property is changed.

- awakeFromFetch

Provides an opportunity to add code into the life cycle of the managed object when it is being fulfilled from a fault.

- awakeFromInsert

Provides an opportunity to add code into the life cycle of the managed object when it is initially created.

- awakeFromSnapshotEvents:

Provides an opportunity to add code into the life cycle of the managed object when it is being fulfilled from a snapshot.

- changedValues

Returns a dictionary containing the keys and (old) values of persistent properties that have been changed since the last fetching or saving of the managed object.

- changedValuesForCurrentEvent

Returns a dictionary containing the keys and old values of persistent properties that have changed since the last posting of NSManagedObjectContextObjectsDidChangeNotification.

- committedValuesForKeys:

Returns a dictionary of the last fetched or saved values of the managed object for the properties specified by the given keys.

- prepareForDeletion

Provides an opportunity to add code into the life cycle of the managed object when it is about to be deleted.

- willSave

Provides an opportunity to add code into the life cycle of the managed object when it is about to be saved.

- didSave

Provides an opportunity to add code into the life cycle of the managed object after the managed object’s context completes a save operation.

- willTurnIntoFault

Provides an opportunity to add code into the life cycle of the managed object before it is converted to a fault.

- didTurnIntoFault

Provides an opportunity to add code into the life cycle of the managed object when it is turned into a fault.

+ fetchRequest

Returns a new fetch request initialized with the entity represented by this subclass.

Supporting Key-Value Coding

- valueForKey:

Returns the value for the property specified by key.

- setValue:forKey:

Sets the specified property of the managed object to the specified value.

- mutableSetValueForKey:

Returns a mutable set that provides read-write access to the unordered to-many relationship specified by a given key.

- primitiveValueForKey:

Returns the value for the specified property from the managed object’s private internal storage .

- setPrimitiveValue:forKey:

Sets the value of a given property in the managed object's private internal storage.

- objectIDsForRelationshipNamed:

Returns the object IDs for all of the managed objects that are in the named relationship.

Managing Data Validation

- validateValue:forKey:error:

Validates a property value for a given key.

- validateForDelete:

Determines whether the managed object can be deleted in its current state.

- validateForInsert:

Determines whether the managed object can be inserted in its current state.

- validateForUpdate:

Determines whether the managed object's current state is valid.

Validation Error Codes

Error codes related to validation of managed object properties.

NSValidationKeyErrorKey

The error key for the attribute that failed to validate.

NSValidationObjectErrorKey

The error key for the object that failed to validate.

NSValidationPredicateErrorKey

The error key for the predicate that failed to validate.

NSValidationValueErrorKey

The error key for the value that failed to validate.

Supporting Key-Value Observing

+ automaticallyNotifiesObserversForKey:

Returns a Boolean value that indicates whether the receiver provides automatic support for key-value observing change notifications for the given key.

- didAccessValueForKey:

Provides support for key-value observing access notification.

- observationInfo

Returns the observation info of the managed object.

- setObservationInfo:

Sets the observation info of the managed object.

- willAccessValueForKey:

Provides support for key-value observing access notification.

- didChangeValueForKey:

Provides an opportunity to respond when a value of a given property has changed.

- didChangeValueForKey:withSetMutation:usingObjects:

Provides an opportunity to respond when a change was made to a specified to-many relationship.

- willChangeValueForKey:

Provides an opportunity to respond when a value of a given property is about to change.

- willChangeValueForKey:withSetMutation:usingObjects:

Provides an opportunity to respond when a change is about to be made to a specified to-many relationship.

Reinitializing Values

NSSnapshotEventType

Constants that specify the reason the managed object may need to reinitialize its values.

Relationships

Inherits From

See Also

Managed Objects and Entities

NSEntityDescription

A description of an entity in Core Data.