Integrated primaryKey extension to NSEntityDescription and refactored cache strategy

classes to eliminate issues with duplicated objects. closes #611, #612, #613, #618

* NSEntityDescription is now aware of the primaryKeyAttribute. Can be configured via
Interface Builder within Xcode or programatically.
* Added findByPrimaryKey: interface to the Core Data extensions.
* Relaxed dependencies on RKManagedObjectMapping across the system now that primaryKey is
available without a reference to the mapping.

Code/CoreData/RKFetchRequestManagedObjectCache.m

@@ -0,0 +1,44 @@
+//
+//  RKFetchRequestMappingCache.m
+//  RestKit
+//
+//  Created by Jeff Arena on 1/24/12.
+//  Copyright (c) 2012 RestKit. All rights reserved.
+//
+
+#import "RKFetchRequestManagedObjectCache.h"
+#import "NSManagedObject+ActiveRecord.h"
+#import "NSEntityDescription+RKAdditions.h"
+#import "RKLog.h"
+
+// Set Logging Component
+#undef RKLogComponent
+#define RKLogComponent lcl_cRestKitCoreData
+
+@implementation RKFetchRequestManagedObjectCache
+
+- (NSManagedObject *)findInstanceOfEntity:(NSEntityDescription *)entity
+                  withPrimaryKeyAttribute:(NSString *)primaryKeyAttribute
+                                    value:(id)primaryKeyValue
+                   inManagedObjectContext:(NSManagedObjectContext *)managedObjectContext
+{
+    NSAssert(entity, @"Cannot find existing managed object without a target class");
+    NSAssert(primaryKeyAttribute, @"Cannot find existing managed object instance without mapping that defines a primaryKeyAttribute");
+    NSAssert(primaryKeyValue, @"Cannot find existing managed object by primary key without a value");
+    NSAssert(managedObjectContext, @"Cannot find existing managed object with a context");
+
+    NSFetchRequest* fetchRequest = [[NSFetchRequest alloc] init];
+    [fetchRequest setEntity:entity];
+    [fetchRequest setFetchLimit:1];
+    [fetchRequest setPredicate:[NSPredicate predicateWithFormat:@"%K = %@", primaryKeyAttribute, primaryKeyValue]];
+    NSArray *objects = [NSManagedObject executeFetchRequest:fetchRequest];
+    RKLogDebug(@"Found objects '%@' using fetchRequest '%@'", objects, fetchRequest);
+
+    NSManagedObject *object = nil;
+    if ([objects count] > 0) {
+        object = [objects objectAtIndex:0];
+    }
+    return object;
+}
+
+@end