diff --git a/lib/active_model/serializer/associations.rb b/lib/active_model/serializer/associations.rb
index e2e13c22..d647d816 100644
--- a/lib/active_model/serializer/associations.rb
+++ b/lib/active_model/serializer/associations.rb
@@ -80,6 +80,11 @@ module ActiveModel
       end
 
       class HasOne < Base #:nodoc:
+        def initialize(name, options={}, serializer_options={})
+          super
+          @polymorphic = options[:polymorphic]
+        end
+
         def root
           if root = options[:root]
             root
@@ -132,14 +137,13 @@ module ActiveModel
 
         private
 
+        attr_reader :polymorphic
+        alias polymorphic? polymorphic
+
         def use_id_key?
           embed_ids? && !polymorphic?
         end
 
-        def polymorphic?
-          options[:polymorphic]
-        end
-
         def polymorphic_key
           object.class.to_s.demodulize.underscore.to_sym
         end