Compute only requested attributes.

lib/active_model/serializer.rb

@@ -238,10 +238,9 @@ module ActiveModel
 
     # Return the +attributes+ of +object+ as presented
     # by the serializer.
-    def attributes
-      attributes = self.class._attributes.dup
-
-      attributes.each_with_object({}) do |name, hash|
+    def attributes(requested_attrs = nil)
+      self.class._attributes.each_with_object({}) do |name, hash|
+        next unless requested_attrs.nil? || requested_attrs.include?(name)
         if self.class._fragmented
           hash[name] = self.class._fragmented.public_send(name)
         else

lib/active_model/serializer/adapter/attributes.rb

@@ -57,10 +57,7 @@ module ActiveModel
 
         def resource_object_for(options)
           cache_check(serializer) do
-            attributes = serializer.attributes
-            attributes.slice!(*options[:fields]) if options[:fields]
-
-            attributes
+            serializer.attributes(options[:fields])
           end
         end
       end

lib/active_model/serializer/adapter/json_api.rb

@@ -140,8 +140,7 @@ module ActiveModel
           cache_check(serializer) do
             resource_object = resource_identifier_for(serializer)
             requested_fields = fieldset && fieldset.fields_for(resource_object[:type])
-            attributes = serializer.attributes.except(:id)
-            attributes.slice!(*requested_fields) if requested_fields
+            attributes = serializer.attributes(requested_fields).except(:id)
             resource_object[:attributes] = attributes if attributes.any?
             resource_object
           end