Inherits from RKObjectMapping : RKMapping : NSObject
Declared in RKEntityMapping.h

Overview

RKEntityMapping objects model an object mapping with a Core Data destination entity.

Entity Identification

One of the fundamental problems when object mapping representations into Core Data entities is determining if a new object should be created or an existing object should be updated. In an entity mapping, one or more attributes can be designated as being used for identification purposes via the identificationAttributes property. Typically the values of these attributes are populated by attribute mappings. It is common practice to use a single attribute corresponding to the primary key of the remote resource being mapped, but an arbitrary number of attributes may be specified for identification. Identifying attributes have all type transformations support by the mapper applied before the managed object context is searched, supporting such use-cases as using an NSDate as an identifying attribute whose value is mapped from an NSString. Identified objects can be further constrained by configuring an identification predicate via the identificationPredicate property. The predicate is applied after the managed object has been searched.

Identification Inference

The RKEntityMapping class provides support for inferring identification attributes from the managed object model. When inference is enabled (the default state), the entity is searched for several commonly used identifying attributes and if any is found, the value of the identificationAttributes property is automatically configured. Inference is performed by the RKIdentificationAttributesInferredFromEntity function.

When RKIdentificationAttributesInferredFromEntity is invoked, the entity is first checked for a user info key specifying the identifying attributes. If the user info of the given entity contains a value for the key ‘RKEntityIdentificationAttributes’, then that value is used to construct an array of attributes. The user info key must contain a string or an array of strings specifying the names of attributes that exist in the given entity.

If no attributes are specified in the user info, then the entity is searched for an attribute whose name matches the llama-cased or snake-cased name of the entity. For example, an entity named ‘Article’ would have an inferred identifying attributes of ‘articleID’ and ‘article_id’, and an entity named ‘ApprovedComment’ would be inferred as ‘approvedCommentID’ and ‘approved_comment_id’. If such an attribute is found within the entity, an array is returned containing the attribute. If none is returned, the the attributes are searched for the following names:

  1. ‘identifier’
  2. ‘id’
  3. ‘ID’
  4. ‘URL’
  5. ‘url’

If any of these attributes are found, then an array is returned containing the attribute. If all possible inferred attributes are exhausted, then nil is returned.

Note that inference will only return a single attribute. Compound attributes must be configured manually via the identificationAttributes property.

Connecting Relationships

When modeling an API into Core Data representation, a common problem is that managed objects that are semantically related are loaded across discrete requests, leaving the Core Data relationships empty. The RKConnectionDescription class provides a means for expressing a connection between entities using corresponding attribute values or by key path. Please refer to the documentation accompanying the RKConnectionDescription class and the addConnectionForRelationship:connectedBy: method of this class.

Tasks

Initializing an Entity Mapping

Accessing the Entity

  •   entity

    The Core Data entity description used for this object mapping

    property

Configuring Managed Object Identification

  •   identificationAttributes

    The array of NSAttributeDescription objects specifying the attributes of the receiver’s entity that are used during mapping to determine whether an existing object should be updated or a new managed object should be inserted. Please see the “Entity Identification” section of this document for more information.

    property
  •   identificationPredicate

    An optional predicate used to filter identified objects during mapping.

    property
  •   modificationKey

    An optional key on objects mapped with the receiver that identifies a property that can be used to detect modification to the instance. This is used to improve the performance of mapping operations by skipping the property mappings for a given object.

    property

Specifying a Persistent Store for Newly Inserted Objects

  •   persistentStore

    The persistent store in which new object instances mapped with the receiver should be inserted.

    property

Configuring Relationship Connections

Configuring Validation

  •   discardsInvalidObjectsOnInsert

    A Boolean value that determines if newly created NSManagedObject instances mapped with the receiver should be discarded when they fail validateForInsert:.

    property

Flagging Objects for Deletion

  •   deletionPredicate

    A predicate that identifies objects for the receiver’s entity that are to be deleted from the local store.

    property

Retrieving Default Attribute Values

  • – defaultValueForAttribute:

    Returns the default value for the specified attribute as expressed in the Core Data entity definition. This value will be assigned if the object mapping is applied and a value for a missing attribute is not present in the payload.

Configuring Entity Identification Inference

Properties

connections

Returns the array of RKConnectionDescripton objects configured for connecting relationships during object mapping.

@property (nonatomic, copy, readonly) NSArray *connections

Declared In

RKEntityMapping.h

deletionPredicate

A predicate that identifies objects for the receiver’s entity that are to be deleted from the local store.

@property (nonatomic, copy) NSPredicate *deletionPredicate

Discussion

This property provides support for local deletion of managed objects mapped as a ‘tombstone’ record from the source representation. The deletion predicate is used in conjunction with the entity associated with the receiver to construct an NSFetchRequest that identifies managed objects that should be deleted when a mapping operation is committed. For example, given the following JSON:

{ "userID": 12345, "is_deleted": true }

We could map the is_deleted key to a Boolean attribute on the model such as shouldBeDeleted and configure a deletion predicate using this attribute:

[entityMapping addAttributeMappingsFromDictionary:@{ @"is_deleted": @"shouldBeDeleted" }];
entityMapping.deletionPredicate = [NSPredicate predicateWithFormat:@"shouldBeDeleted = true"];

When a mapping operation completes, a NSFetchRequest will be constructed and executed. Any objects in the store whose shouldBeDeleted value is true will be deleted.

Declared In

RKEntityMapping.h

discardsInvalidObjectsOnInsert

A Boolean value that determines if newly created NSManagedObject instances mapped with the receiver should be discarded when they fail validateForInsert:.

@property (nonatomic, assign) BOOL discardsInvalidObjectsOnInsert

Discussion

This property allows for the deletion of managed objects that fail validation such that NSManagedObjectContext save will complete successfully. Typically an invalid managed object in the graph will result in a failure to save the NSManagedObjectContext due to an NSValidation error. In some cases it is desirable to persist only the subset of objects that pass validation and discard the invalid content rather than failing the entire operation. Setting this property to YES will result in the deletion of in any newly created NSManagedObject instances that fail to return YES when sent the validateForInsert: message.

Default: NO

Declared In

RKEntityMapping.h

entity

The Core Data entity description used for this object mapping

@property (nonatomic, strong) NSEntityDescription *entity

Declared In

RKEntityMapping.h

identificationAttributes

The array of NSAttributeDescription objects specifying the attributes of the receiver’s entity that are used during mapping to determine whether an existing object should be updated or a new managed object should be inserted. Please see the “Entity Identification” section of this document for more information.

@property (nonatomic, copy) NSArray *identificationAttributes

Return Value

An array of identifying attributes or nil if none have been configured. @raises NSInvalidArgumentException Raised if the setter is invoked with the name of an attribute or an NSAttributeDescription that does not exist in the receiver’s entity. Also raised if the setter is invoked with an empty array.

Discussion

Warning: Note that for convenience, this property may be set with an array containing NSAttributeDescription objects or NSString objects specifying the names of attributes that exist within the receiver’s entity. The getter will always return an array of NSAttributeDescription objects.

Declared In

RKEntityMapping.h

identificationPredicate

An optional predicate used to filter identified objects during mapping.

@property (nonatomic, copy) NSPredicate *identificationPredicate

Return Value

The identification predicate.

Declared In

RKEntityMapping.h

modificationKey

An optional key on objects mapped with the receiver that identifies a property that can be used to detect modification to the instance. This is used to improve the performance of mapping operations by skipping the property mappings for a given object.

@property (nonatomic, copy) NSString *modificationKey

Discussion

A common modification key is a ‘last modified’ or ‘updated at’ timestamp that specifies the last change to an object. When the modificationKey is non-nil, the mapper will compare the value returned for the key on an existing object instance with the value in the representation being mapped. If they are exactly equal, then the mapper will skip all remaining property mappings and proceed to the next object.

Declared In

RKEntityMapping.h

persistentStore

The persistent store in which new object instances mapped with the receiver should be inserted.

@property (nonatomic, weak) NSPersistentStore *persistentStore

Discussion

If your application makes use of more than one persistent store (i.e. a combination of an in-memory store and a SQLite store), then it can be desirable to specify the persistent store in which newly created managed objects will be assigned.

Default: nil

Declared In

RKEntityMapping.h

Class Methods

isEntityIdentificationInferenceEnabled

Returns a Boolean value that indicates if entity identification inference has been enabled.

+ (BOOL)isEntityIdentificationInferenceEnabled

Return Value

YES if entity identification inference is enabled, else NO.

Declared In

RKEntityMapping.h

mappingForEntityForName:inManagedObjectStore:

A convenience initializer that creates and returns an entity mapping for the entity with the given name in the managed object model of the given managed object store.

+ (instancetype)mappingForEntityForName:(NSString *)entityName inManagedObjectStore:(RKManagedObjectStore *)managedObjectStore

Parameters

entityName

The name of the entity in the managed object model for which an entity mapping is to be created.

managedObjectStore

A managed object store containing the managed object model in which an entity with the given name is defined.

Return Value

A new entity mapping for the entity with the given name in the managed object model of the given managed object store.

Discussion

This method is functionally equivalent to the following example code:

 NSEntityDescription *entity = [[managedObjectStore.managedObjectModel entitiesByName] objectForKey:entityName];
 return [RKEntityMapping mappingForEntity:entity];

Declared In

RKEntityMapping.h

setEntityIdentificationInferenceEnabled:

Enables or disabled entity identification inference.

+ (void)setEntityIdentificationInferenceEnabled:(BOOL)enabled

Parameters

enabled

A Boolean value indicating if entity identification inference is to be performed.

Discussion

Default: YES

Declared In

RKEntityMapping.h

Instance Methods

addConnection:

Adds a connection to the receiver.

- (void)addConnection:(RKConnectionDescription *)connection

Parameters

connection

The connection to be added.

Declared In

RKEntityMapping.h

addConnectionForRelationship:connectedBy:

Adds a connection for the specified relationship connected using the attributes specified by the given NSString, NSArray, or NSDictionary object.

- (void)addConnectionForRelationship:(id)relationshipOrName connectedBy:(id)connectionSpecifier

Parameters

relationshipOrName

The relationship object or name of the relationship object that is to be connected.

connectionSpecifier

An NSString, NSArray, or NSDictionary object specifying how the relationship is to be connected by matching attributes.

Discussion

This is a convenience method for flexibly adding a connection to the receiver. The relationship can be specified with by providing an NSRelationshipDescription object or an NSString specifying the name of the relationship to be connected. The connection specifier can be provided as an NSString object, an NSArray of NSString object, or an NSDictionary with NSString objects for the keys and values. The string objects specify the name of attributes within the entity and destination entity of the specified relationship. The RKConnectionDescription class models a connection as a dictionary in which the keys are NSString objects corresponding to the names of attributes in the source entity of the relationship being connected and the values are NSString objects corresponding to the names of attributes in the destination entity.

When the connectionSpecifier is an NSString value, it is interpretted as the name of an attribute in the specified relationship’s entity. The corresponding attribute in the destination entity is determined by invoking the source to destination key transformation block set via [RKObjectMapping setSourceToDestinationKeyTransformationBlock:]. If no transformation block is configured, then the destination attribute is assumed to have the same name as the source attribute. For example, consider a model in which there entities named ‘User’ and ‘Project’. The ‘User’ entity has a to-many relationship to the ‘Project’ entity named ‘projects’. Both the ‘User’ and the ‘Project’ entities contain an attribute named ‘userID’, which represents the value for the primary key of the ‘User’ in the API the application is communicating with. When the user’s projects are loaded from the ‘/projects’ endpoint, the user ID is sent down in the JSON representation of the Project objects as a numeric value. In order to establish a connection for the ‘projects’ relationship between the ‘User’ and ‘Project’ entities, we could add the connection like so:

// JSON looks like {"project": { "name": "Project Name", "userID": 1234, "projectID": 1 } }
RKEntityMapping *mapping = [RKEntityMapping mappingForEntityForName:@"Project" inManagedObjectStore:managedObjectStore];
[mapping addAttributeMappingsFromArray:@[ @"name", @"userID", @"projectID" ]];

// Find a 'User' whose value for the 'userID' object is equal to the value stored on the 'userID' attribute of the 'Project' and assign it to the relationship
// In other words, "Find the User whose userID == 1234 and assign that object to the 'user' relationship"
[mapping addConnectionForRelationship:@"user" connectedBy:@"userID"];

When the connection is attempted to be established by an instance of RKRelationshipConnectionOperation, the value for the ‘userID’ attribute will be read from the Project (in this case, @1234) and the managed object context will be searched for a managed object for the ‘User’ entity with a corresponding value for its ‘userID’ attribute.

When the connectionSpecifier is an NSArray object, it is interpretted as containing the names of attributes in the specified relationship’s entity. Just as with a stirng value, the corresponding destination attributes are determined by invoking the source to destination key transformation block or they are assumed to have matching names.

When the connectionSpecifier is an NSDictionary object, the keys are interpretted as containing the names of attributes on the source entity the values are interpretted as the names of attributes on the destination entity. For example:

// Find the User whose userID is equal to the value stored on the 'createdByUserID' attribute
[mapping addConnectionForRelationship:@"createdByUser" connectedBy:@{ @"createdByUserID": @"userID" }];

See Also

  • [RKObjectMapping setSourceToDestinationKeyTransformationBlock:]

Declared In

RKEntityMapping.h

connectionForRelationship:

Returns the connection for the specified relationship.

- (RKConnectionDescription *)connectionForRelationship:(id)relationshipOrName

Parameters

relationshipOrName

The relationship object or name of the relationship object for which to retrieve the connection.

Return Value

The connection object for the specified relationship or nil if none is configured.

Declared In

RKEntityMapping.h

defaultValueForAttribute:

Returns the default value for the specified attribute as expressed in the Core Data entity definition. This value will be assigned if the object mapping is applied and a value for a missing attribute is not present in the payload.

- (id)defaultValueForAttribute:(NSString *)attributeName

Declared In

RKEntityMapping.h

initWithEntity:

Initializes the receiver with a given entity.

- (id)initWithEntity:(NSEntityDescription *)entity

Parameters

entity

An entity with which to initialize the receiver.

Return Value

The receiver, initialized with the given entity.

Declared In

RKEntityMapping.h

removeConnection:

Removes a connection from the receiver.

- (void)removeConnection:(RKConnectionDescription *)connection

Parameters

connection

The connection to be removed.

Declared In

RKEntityMapping.h