title: Restkit property mapping Failed Transformation error
body: <p>this is what my sample code looks like:</p>
  
  <pre><code>{
      "name": "Ahmad Mansour",
      "subjects": [
          {
              "parent_subject_name": "Arabic",
              "subject_name": "Shafahi",
              "exams": [
                  {
                      "score": "30.00",
                      "exam_name": "Sa3i 1 "
                  },
                  {
                      "score": "50.00",
                      "exam_name": "sa3i 2"
                  },
                  {
                      "score": "100.00",
                      "exam_name": "First Semester Exam"
                  }
              ]
          },
          {
              "parent_subject_name": "Arabic",
              "subject_name": "Khati",
              "exams": [
                  {
                      "score": "50.00",
                      "exam_name": "Sa3i 1 "
                  },
                  {
                      "score": "60.00",
                      "exam_name": "sa3i 2"
                  },
                  {
                      "score": "95.00",
                      "exam_name": "First Semester Exam"
                  }
              ]
          },
  </code></pre>
  
  <p>for the <code>subject</code> entity.. my mapping works just fine:</p>
  
  <pre><code>RKEntityMapping *subjectEntityMapping = [RKEntityMapping mappingForEntityForName:kSubjectIdentity inManagedObjectStore:model.managedObjectStore];
  [subjectEntityMapping addAttributeMappingsFromDictionary:@{ @"subject_name": @"name",
                                                              @"parent_subject_name" :@"parent_name"
                                                            }];
  
  subjectEntityMapping.identificationAttributes = @[ @"name" ];
  
  RKResponseDescriptor *studentResponseDescriptor = [RKResponseDescriptor responseDescriptorWithMapping:subjectEntityMapping pathPattern:kGradesPath keyPath:@"subjects" statusCodes:RKStatusCodeIndexSetForClass(RKStatusCodeClassSuccessful)];
  
  [model.objectManager addResponseDescriptor:studentResponseDescriptor];
  </code></pre>
  
  <p>but when i do the exam score mapping.. things blow up:</p>
  
  <pre><code>RKEntityMapping *examEntityMapping = [RKEntityMapping mappingForEntityForName:kExamIdentity inManagedObjectStore:model.managedObjectStore];
  [examEntityMapping addAttributeMappingsFromDictionary:@{ @"exams.exam_name": @"name" }];
  
  examEntityMapping.identificationAttributes = @[ @"name" ];
  
  RKResponseDescriptor *examResponseDescriptor = [RKResponseDescriptor responseDescriptorWithMapping:examEntityMapping pathPattern:kGradesPath keyPath:@"subjects" statusCodes:RKStatusCodeIndexSetForClass(RKStatusCodeClassSuccessful)];
  
  [model.objectManager addResponseDescriptor:examResponseDescriptor];
  </code></pre>
  
  <p>i get the following error:</p>
  
  <pre><code> E restkit.object_mapping:RKMappingOperation.m:431 Failed transformation of value at keyPath 'exams.exam_name' to representation of type 'NSString': Error Domain=org.restkit.RKValueTransformers.ErrorDomain Code=3002 "Failed transformation of value '(
      "Sa3i 1 ",
      "sa3i 2",
      "First Semester Exam"
  )' to NSString: none of the 2 value transformers consulted were successful." UserInfo=0x9a508a0 {detailedErrors=(
      "Error Domain=org.restkit.RKValueTransformers.ErrorDomain Code=3002 \"The given value is not already an instance of 'NSString'\" UserInfo=0x9a50800 {NSLocalizedDescription=The given value is not already an instance of 'NSString'}",
      "Error Domain=org.restkit.RKValueTransformers.ErrorDomain Code=3000 \"Expected an `inputValue` of type `NSNull`, but got a `__NSArrayI`.\" UserInfo=0x9a50830 {NSLocalizedDescription=Expected an `inputValue` of type `NSNull`, but got a `__NSArrayI`.}"
  </code></pre>
  
  <p>I also tried this mapping, but i get the exact same error:</p>
  
  <pre><code>[examEntityMapping addAttributeMappingsFromDictionary:@{ @"exam_name": @"name" }];
  
  examEntityMapping.identificationAttributes = @[ @"name" ];
  
  RKResponseDescriptor *examResponseDescriptor = [RKResponseDescriptor responseDescriptorWithMapping:examEntityMapping pathPattern:kGradesPath keyPath:@"subjects.exams" statusCodes:RKStatusCodeIndexSetForClass(RKStatusCodeClassSuccessful)];
  </code></pre>
  
  <p>ideas?</p>