Added class function find, to allow finding models by primary key

spec/javascripts/resourceSpec.js

@@ -41,7 +41,7 @@ describe('A Resource instance', function() {
 
     it('should not have a URL', function() {
       expect(model.resourceURL()).toBeUndefined();
-    })
+    });
   });
 
   it('allows setting of properties not in the schema during creation', function() {
@@ -80,4 +80,18 @@ describe('A Resource instance', function() {
     });
   });
 
+  describe('finding objects via find', function() {
+      beforeEach(function() {
+          model = Model.create({id: 1});
+      });
+
+      it('should return model for the given primary key if it exists', function() {
+          expect(Model.find(1)).toBe(model);
+      });
+
+      it('should return null if no model with the given primary key exists', function() {
+          expect(Model.find(2)).toBeNull();
+      });
+  });
+
 });

src/sproutcore-resource.js

@@ -907,6 +907,13 @@
       return this;
     },
 
+    find: function(id) {
+        if (SC.none(this.identityMap)) {
+            return null;
+        }
+        return this.identityMap[id] || null;
+    },
+
     // Create an instance of this resource. If `options` includes an
     // `id`, first check the identity map and return the existing resource
     // with that ID if found.