add class attribute :root to ArraySerializer

You can now set the default behavior for Array serialization in a
single place

lib/action_controller/serialization.rb

@@ -40,13 +40,17 @@ module ActionController
     end
 
     def _render_option_json(json, options)
-      if json.respond_to?(:to_ary)
-        options[:root] ||= controller_name unless options[:root] == false
-      end
-
       serializer = options.delete(:serializer) ||
         (json.respond_to?(:active_model_serializer) && json.active_model_serializer)
 
+      if json.respond_to?(:to_ary)
+        if options[:root] != false && serializer.root != false
+          # default root element for arrays is serializer's root or the controller name
+          # the serializer for an Array is ActiveModel::ArraySerializer
+          options[:root] ||= serializer.root || controller_name
+        end
+      end
+
       if serializer
         options[:scope] = serialization_scope
         options[:url_options] = url_options

lib/active_model/serializer.rb

@@ -29,11 +29,18 @@ module ActiveModel
 
   # Active Model Array Serializer
   #
-  # It serializes an array checking if each element that implements
+  # It serializes an Array, checking if each element that implements
   # the +active_model_serializer+ method.
+  #
+  # To disable serialization of root elements, in an initializer:
+  #
+  #     ActiveModel::ArraySerializer.root = false
+  #
   class ArraySerializer
     attr_reader :object, :options
 
+    class_attribute :root
+
     def initialize(object, options={})
       @object, @options = object, options
     end

test/serialization_test.rb

@@ -66,6 +66,10 @@ class RenderJsonTest < ActionController::TestCase
     end
   end
 
+  class CustomArraySerializer < ActiveModel::ArraySerializer
+    self.root = "items"
+  end
+
   class TestController < ActionController::Base
     protect_from_forgery
 
@@ -144,6 +148,14 @@ class RenderJsonTest < ActionController::TestCase
       render :json => [], :root => false
     end
 
+    def render_json_empty_array
+      render :json => []
+    end
+
+    def render_json_array_with_custom_array_serializer
+      render :json => [], :serializer => CustomArraySerializer
+    end
+
 
   private
     def default_serializer_options
@@ -260,4 +272,23 @@ class RenderJsonTest < ActionController::TestCase
     get :render_json_array_with_no_root
     assert_equal '[]', @response.body
   end
+
+  def test_render_json_empty_array
+    get :render_json_empty_array
+    assert_equal '{"test":[]}', @response.body
+  end
+
+  def test_render_json_empty_arry_with_array_serializer_root_false
+    ActiveModel::ArraySerializer.root = false
+    get :render_json_empty_array
+    assert_equal '[]', @response.body
+  ensure # teardown
+    ActiveModel::ArraySerializer.root = nil
+  end
+
+  def test_render_json_array_with_custom_array_serializer
+    get :render_json_array_with_custom_array_serializer
+    assert_equal '{"items":[]}', @response.body
+  end
+
 end