Add objectMapping identity attribute on RKPropertyMapping to allow one to traverse the mapping graph from an attribute/relationship back to the parent mapping

Code/ObjectMapping/RKObjectMapping.m

@@ -33,6 +33,10 @@ static NSUInteger RKObjectMappingMaximumInverseMappingRecursionDepth = 100;
 // Private declaration
 NSDate *RKDateFromStringWithFormatters(NSString *dateString, NSArray *formatters);
 
+@interface RKPropertyMapping ()
+@property (nonatomic, weak, readwrite) RKObjectMapping *objectMapping;
+@end
+
 @interface RKObjectMapping ()
 @property (nonatomic, weak, readwrite) Class objectClass;
 @property (nonatomic, strong) NSMutableArray *mutablePropertyMappings;
@@ -146,6 +150,8 @@ NSDate *RKDateFromStringWithFormatters(NSString *dateString, NSArray *formatters
     NSAssert1([[self mappedKeyPaths] containsObject:propertyMapping.destinationKeyPath] == NO,
               @"Unable to add mapping for keyPath %@, one already exists...", propertyMapping.destinationKeyPath);
     NSAssert(self.mutablePropertyMappings, @"self.mutablePropertyMappings is nil");
+    NSAssert(propertyMapping.objectMapping == nil, @"Cannot add a property mapping that already exists in another mapping.");
+    propertyMapping.objectMapping = self;
     [self.mutablePropertyMappings addObject:propertyMapping];
 }
 
@@ -212,7 +218,10 @@ NSDate *RKDateFromStringWithFormatters(NSString *dateString, NSArray *formatters
 
 - (void)removePropertyMapping:(RKPropertyMapping *)attributeOrRelationshipMapping
 {
-    [self.mutablePropertyMappings removeObject:attributeOrRelationshipMapping];
+    if ([self.mutablePropertyMappings containsObject:attributeOrRelationshipMapping]) {
+        attributeOrRelationshipMapping.objectMapping = nil;
+        [self.mutablePropertyMappings removeObject:attributeOrRelationshipMapping];
+    }
 }
 
 - (RKObjectMapping *)inverseMappingAtDepth:(NSInteger)depth

Code/ObjectMapping/RKPropertyMapping.h

@@ -20,11 +20,22 @@
 
 #import <Foundation/Foundation.h>
 
+@class RKObjectMapping;
+
 /**
  `RKPropertyMapping` is an abstract class for describing the properties being mapped within an `RKObjectMapping` or `RKEntityMapping` object. It defines the common interface for its concrete subclasses `RKAttributeMapping` and `RKRelationshipMapping`. Each property mapping defines a single transformation from a source key path (often in the deserialized representation of a JSON or XML document) to a destination key path (typically on a target object).
  */
 @interface RKPropertyMapping : NSObject <NSCopying>
 
+///------------------------------------------
+/// @name Accessing the Parent Object Mapping
+///------------------------------------------
+
+/**
+ Returns the object mapping the receiver is added to.
+ */
+@property (nonatomic, weak, readonly) RKObjectMapping *objectMapping;
+
 ///-----------------------------------------------------
 /// @name Accessing the Source and Destination Key Paths
 ///-----------------------------------------------------

Code/ObjectMapping/RKPropertyMapping.m

@@ -5,10 +5,24 @@
 //  Created by Blake Watters on 8/27/12.
 //  Copyright (c) 2012 RestKit. All rights reserved.
 //
+//  Licensed under the Apache License, Version 2.0 (the "License");
+//  you may not use this file except in compliance with the License.
+//  You may obtain a copy of the License at
+//
+//  http://www.apache.org/licenses/LICENSE-2.0
+//
+//  Unless required by applicable law or agreed to in writing, software
+//  distributed under the License is distributed on an "AS IS" BASIS,
+//  WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+//  See the License for the specific language governing permissions and
+//  limitations under the License.
+//
 
 #import "RKPropertyMapping.h"
 
 @interface RKPropertyMapping ()
+// Synthesize as read/write to allow assignment in `RKObjectMapping`
+@property (nonatomic, weak, readwrite) RKObjectMapping *objectMapping;
 @property (nonatomic, strong, readwrite) NSString *sourceKeyPath;
 @property (nonatomic, strong, readwrite) NSString *destinationKeyPath;
 @end