Refactor adapters to implement support for array serialization

lib/active_model/serializer/adapter/json.rb

@@ -3,18 +3,26 @@ module ActiveModel
     class Adapter
       class Json < Adapter
         def serializable_hash(options = {})
-          @hash = serializer.attributes(options)
+          if serializer.respond_to?(:each)
+            @result = serializer.map{|s| self.class.new(s).serializable_hash }
+          else
+            @result = serializer.attributes(options)
 
-          serializer.each_association do |name, association, options|
-            if association.respond_to?(:each)
-              array_serializer = association
-              @hash[name] = array_serializer.map { |item| item.attributes(options) }
-            else
-              @hash[name] = association.attributes(options)
+            serializer.each_association do |name, association, opts|
+              if association.respond_to?(:each)
+                array_serializer = association
+                @result[name] = array_serializer.map { |item| item.attributes(opts) }
+              else
+                @result[name] = association.attributes(options)
+              end
             end
           end
 
-          @hash
+          if root = options.fetch(:root, serializer.json_key)
+            @result = { root => @result }
+          end
+
+          @result
         end
       end
     end