RestKit Entity mapping - ios

Question

Hi can someone teach me how to do nested Restkit Entity Mapping? i keep get error message while debug, below are my error message and code
[__NSSetM insertObject:atIndex:]: unrecognized selector sent to instance 0x95269d0
Json data
Family =(
{
id = "1";
parentName = "Mr John";
Child =(
{
parentID = "1";
childName = "James";
age = "18";
},
{
parentID = "1";
childName = "ruby";
age = "19";
},
{
parentID = "1";
childName = "ella";
age = "20";
}
);
}
);
My AppDelegate.m
RKEntityMapping *familyMapping = [RKEntityMapping mappingForEntityForName:#"Family" inManagedObjectStore:managedObjectStore];
debtorMapping.identificationAttributes = #[ #"id" ];
[familyMapping addAttributeMappingsFromDictionary:#{
#"id": #"accNo",
#"parentName": #"companyName"
}];
RKEntityMapping *childMapping = [RKEntityMapping mappingForEntityForName:#"Child" inManagedObjectStore:managedObjectStore];
childMapping.identificationAttributes = #[ #"parentID"];
[childMapping addAttributeMappingsFromDictionary:#{
#"parentID": #"parentID",
#"childName": #"childName",
#"age": #"age"
}];
[familyMapping addPropertyMapping:[RKRelationshipMapping relationshipMappingFromKeyPath:#"Child" toKeyPath:#"Child" withMapping:childMapping]];
RKResponseDescriptor *responseDescriptor = [RKResponseDescriptor responseDescriptorWithMapping:familyMapping
pathPattern:nil
keyPath:#"Family"
statusCodes:RKStatusCodeIndexSetForClass(RKStatusCodeClassSuccessful)];
[objectManager addResponseDescriptor:responseDescriptor];
Family NSManagedObject
#class Child;
#interface Family : NSManagedObject
#property (nonatomic, retain) NSString * id;
#property (nonatomic, retain) NSString * parentName;
#property (nonatomic, retain) NSSet *child;
#end
#interface Family (CoreDataGeneratedAccessors)
- (void)addChildObject:(Child *)value;
- (void)removeChildObject:(Child *)value;
- (void)addChild:(NSSet *)values;
- (void)removeChild:(NSSet *)values;
#end
Child NSManageObject
#class Family;
#interface Child : NSManagedObject
#property (nonatomic, retain) NSString * parentID;
#property (nonatomic, retain) NSString * childName;
#property (nonatomic, retain) NSString * age;
#property (nonatomic, retain) Family *family;
#end

Answer 1

i found out my mistake already.
[familyMapping addPropertyMapping:[RKRelationshipMapping relationshipMappingFromKeyPath:#"Child" toKeyPath:#"child" withMapping:childMapping]];
the toKeyPath string should assign with small letter. "follow core data relationship string".