Inherits from RKPropertyMapping : NSObject
Declared in RKRelationshipMapping.h


The RKRelationshipMapping class is used to describe relationships of a class in an RKObjectMapping or an entity in an RKEntityMapping object.

RKRelationshipMapping extends RKPropertyMapping to describe features specific to relationships, including the RKMapping object describing how to map the destination object.

Relationship mappings are described in terms of a source key path, which identifies a key in the parent object representation under which the data for the relationship is nested, and a destination key path, which specifies the key path at which the mapped object is to be assigned on the parent entity. The key paths of the property mappings of the RKMapping object in the relationship mapping are evaluated against the nested object representationship at the source key path.

Mapping a Non-nested Relationship from the Parent Representation

It can often be desirable to map data for a relationship directly from the parent object representation, rather than under a nested key path. When a relationship mapping is constructed with a nil value for the source key path, then the RKMapping object is evaluated against the parent representation.

Assignment Policy

When mapping a relationship, the typical desired behavior is to set the destination of the relationship to the newly mapped values from the object representation being processed. There are times in which it is desirable to use different assignment behaviors. The way in which the relationship is assigned can be controlled by the assignmentPolicy property. There are currently three distinct assignment policies available:

  1. RKSetAssignmentPolicy - Instructs the mapper to assign the new destination value to the relationship directly. No further action is taken and the relationship to the old objects is broken. This is the default assignment policy.
  2. RKReplaceAssignmentPolicy - Instructs the mapper to assign the new destination value to the relationship and delete any existing object or objects at the destination. The deletion behavior is contextual based on the type of objects being mapped (i.e. Core Data vs NSObject) and is delegated to the mapping operation data source.
  3. RKUnionAssignmentPolicy - Instructs the mapper to build a new value for the relationship by unioning the existing value with the new value and set the combined value to the relationship. The union assignment policy is only appropriate for use with a to-many relationship.


Creating a Relationship Mapping

Accessing the Destination Mapping

  •   mapping

    An RKMapping object describing how to map the object representation at sourceKeyPath to a new represenation at destinationKeyPath.


Configuring the Assignment Policy



@property (nonatomic, assign) RKAssignmentPolicy assignmentPolicy

The assignment policy to use when applying the relationship mapping.

The assignment policy determines how a relationship is set when there are existing objects at the destination of the relationship. The existing values can be disconnected from the parent and left in the graph (RKSetAssignmentPolicy), deleted and replaced by the new value (RKReplaceAssignmentPolicy), or the new value can be unioned with the existing objects to create a new combined value (RKUnionAssignmentPolicy).

Default: RKSetAssignmentPolicy

Declared In



@property (nonatomic, strong, readonly) RKMapping *mapping

An RKMapping object describing how to map the object representation at sourceKeyPath to a new represenation at destinationKeyPath.

Declared In


Class Methods


+ (instancetype)relationshipMappingFromKeyPath:(NSString *)sourceKeyPath toKeyPath:(NSString *)destinationKeyPath withMapping:(RKMapping *)mapping

Creates and returns a new relationship mapping object describing how to transform a related object representation at sourceKeyPath to a new representation at destinationKeyPath using the given mapping.

The mapping may describe a to-one or a to-many relationship. The appropriate handling of the source representation is deferred until run-time and is determined by performing reflection on the data retrieved from the source object representation by sending a valueForKeyPath: message where the key path is the value given in sourceKeyPath. If an NSArray, NSSet or NSOrderedSet object is returned, the related object representation is processed as a to-many collection. Otherwise the representation is considered to be a to-one.



A key path from which to retrieve data in the source object representation that is to be mapped as a relationship. If nil, then the mapping is performed directly against the parent object representation.


The key path on the destination object to set the object mapped results.


A mapping object describing how to map the data retrieved from sourceKeyPath that is to be set on destinationKeyPath.

Declared In