Extract attributes filtering from serializer into adapter.

lib/active_model/serializer/fieldset.rb

@@ -1,8 +1,7 @@
 module ActiveModel
   class Serializer
     class Fieldset
-      def initialize(fields, root = nil)
-        @root       = root
+      def initialize(fields)
         @raw_fields = fields
       end
 
@@ -10,27 +9,19 @@ module ActiveModel
         @fields ||= parsed_fields
       end
 
-      def fields_for(serializer)
-        key = serializer.json_key
-        fields[key.to_sym] || fields[key.pluralize.to_sym]
+      def fields_for(type)
+        fields[type.singularize.to_sym] || fields[type.pluralize.to_sym]
       end
 
       private
 
       ActiveModelSerializers.silence_warnings do
-        attr_reader :raw_fields, :root
+        attr_reader :raw_fields
       end
 
       def parsed_fields
         if raw_fields.is_a?(Hash)
           raw_fields.inject({}) { |h, (k, v)| h[k.to_sym] = v.map(&:to_sym); h }
-        elsif raw_fields.is_a?(Array)
-          if root.nil?
-            raise ArgumentError, 'The root argument must be specified if the fields argument is an array.'.freeze
-          end
-          hash = {}
-          hash[root.to_sym] = raw_fields.map(&:to_sym)
-          hash
         else
           {}
         end