Let FlattenJson adapter decide it doesn't include meta

lib/active_model/serializer/adapter.rb

@@ -22,7 +22,7 @@ module ActiveModel
 
       def as_json(options = nil)
         hash = serializable_hash(options)
-        include_meta(hash) unless self.class == FlattenJson
+        include_meta(hash)
         hash
       end
 

lib/active_model/serializer/adapter/flatten_json.rb

@@ -6,6 +6,13 @@ module ActiveModel
           super
           @result
         end
+
+        private
+
+        # no-op: FlattenJson adapter does not include meta data, because it does not support root.
+        def include_meta(json)
+          json
+        end
       end
     end
   end