teach #move_to_child_with_index to accept :root as a target.

lib/awesome_nested_set/model/movable.rb

@@ -41,7 +41,17 @@ def move_to_child_of(node)
 
           # Move the node to the child of another node with specify index
           def move_to_child_with_index(node, index)
-            if node.children.empty?
+            if node == :root
+              move_to_root
+              my_position = self_and_siblings.index(self)
+              if my_position < index
+                move_to_right_of(self_and_siblings[index])
+              elsif my_position && my_position == index
+                # do nothing. already there.
+              else
+                move_to_left_of(self_and_siblings[index])
+              end
+            elsif node.children.empty?
               move_to_child_of(node)
             elsif node.children.count == index
               move_to_right_of(node.children.last)

spec/awesome_nested_set_spec.rb

@@ -562,6 +562,13 @@
       expect(categories(:child_1).left).to eq(2)
       expect(categories(:child_1).right).to eq(3)
     end
+
+    it "move to top level with specified index" do
+      categories(:child_1).move_to_child_with_index(:root, 1)
+      expect(categories(:child_1).parent_id).to be_nil
+      expect(categories(:child_1).left).to eq(9)
+      expect(categories(:child_1).right).to eq(10)
+    end
   end
 
   it "move_to_child_of_appends_to_end" do