Add sort descriptors for properties to a Scope.

app/scope.rb

@@ -38,14 +38,19 @@ def where(conditions, *formatArguments)
                        inContext:@context)
     end
 
-    # Sort ascending by an attribute, or a NSSortDescriptor.
-    def sortBy(attribute)
-      sortBy(attribute, ascending:true)
+    # Sort ascending by a key-path, or a NSSortDescriptor.
+    def sortBy(property)
+      sortBy(property, ascending:true)
     end
 
-    # Sort by an attribute.
-    def sortBy(attribute, ascending:ascending)
-
+    # Sort by a key-path.
+    def sortBy(keyPath, ascending:ascending)
+      sortDescriptors = @sortDescriptors ? @sortDescriptors.dup : []
+      sortDescriptors << NSSortDescriptor.alloc.initWithKey(keyPath.to_s, ascending:ascending)
+      Scope.alloc.initWithTarget(@target,
+                       predicate:@predicate,
+                 sortDescriptors:sortDescriptors,
+                       inContext:@context)
     end
 
     # Factory methods

spec/scope_spec.rb

@@ -1,3 +1,13 @@
+class NSSortDescriptor
+  def inspect
+    description
+  end
+
+  def ==(other)
+    description == other.description
+  end
+end
+
 module MotionData
 
   describe Scope do
@@ -6,9 +16,13 @@ module MotionData
       scope.target.should == Author
       scope.context.should == Context.current
     end
+
+    it "stores a copy of the given sort descriptors" do
+
+    end
   end
 
-  describe Scope, "when building a new scope by applying finder options" do
+  describe Scope, "when building a new scope by adding finder conditions" do
     extend Predicate::Builder::Mixin
 
     it "from a hash" do
@@ -82,4 +96,23 @@ module MotionData
     end
   end
 
+  describe Scope, "when building a new scope by adding sort conditions" do
+    extend Predicate::Builder::Mixin
+
+    it "sorts by a property" do
+      scope1 = Scope.alloc.initWithTarget(Author)
+
+      scope2 = scope1.sortBy(:name, ascending:true)
+      scope2.object_id.should.not == scope1.object_id
+      scope2.sortDescriptors.should == [NSSortDescriptor.alloc.initWithKey('name', ascending:true)]
+
+      scope3 = scope2.sortBy(:amount, ascending:false)
+      scope3.object_id.should.not == scope2.object_id
+      scope3.sortDescriptors.should == [
+        NSSortDescriptor.alloc.initWithKey('name', ascending:true),
+        NSSortDescriptor.alloc.initWithKey('amount', ascending:false)
+      ]
+    end
+  end
+
 end