creating flatten_json adapter

lib/active_model/serializer/adapter.rb

@@ -82,7 +82,7 @@ module ActiveModel
       end
 
       def root
-        @options.fetch(:root) { serializer.json_key }
+        serializer.json_key
       end
 
       def include_meta(json)

lib/active_model/serializer/adapter/flatten_json.rb

@@ -0,0 +1,18 @@
+require 'active_model/serializer/adapter/json/fragment_cache'
+
+module ActiveModel
+  class Serializer
+    class Adapter
+      class FlattenJson < Json
+        def serializable_hash(options = {})
+          super
+          @result
+        end
+      end
+
+      def fragment_cache(cached_hash, non_cached_hash)
+        Json::FragmentCache.new().fragment_cache(cached_hash, non_cached_hash)
+      end
+    end
+  end
+end

lib/active_model/serializer/adapter/json.rb

@@ -37,11 +37,7 @@ module ActiveModel
             @result = @core.merge @hash
           end
 
-          if root
-            @result = { root => @result }
-          else
-            @result
-          end
+          { root => @result }
         end
       end