implement sparse fieldsets http://jsonapi.org/format/#fetching-sparse-fieldsets

lib/active_model/serializer/fieldset.rb

@@ -0,0 +1,33 @@
+module ActiveModel
+  class Serializer
+    class Fieldset
+
+      attr_accessor :fields, :root
+
+      def initialize(serializer, fields = {})
+        @root   = serializer.json_key
+        @fields = parse(fields)
+      end
+
+      def fields_for(serializer)
+        key = serializer.json_key || serializer.class.root_name
+        fields[key]
+      end
+
+    private
+
+      def parse(fields)
+        if fields.is_a?(Hash)
+          fields.inject({}) { |h,(k,v)| h[k.to_s] = v.map(&:to_sym); h}
+        elsif fields.is_a?(Array)
+          hash = {}
+          hash[root.to_s] = fields.map(&:to_sym)
+          hash
+        else
+          {}
+        end
+      end
+
+    end
+  end
+end