Fix constant lookup when serializer is not available.

lib/active_model_serializers.rb

@@ -5,14 +5,19 @@ module ActiveModel::SerializerSupport
   extend ActiveSupport::Concern
 
   module ClassMethods #:nodoc:
-    def active_model_serializer
-      return @active_model_serializer if defined?(@active_model_serializer)
-
-      # Use safe constantize when Rails 3.2 is out
-      begin
-        @active_model_serializer = "#{self.name}Serializer".constantize
-      rescue NameError => e
-        raise unless e.message =~ /uninitialized constant$/ && e.name.to_s == "#{self.name}Serializer"
+    if "".respond_to?(:safe_constantize)
+      def active_model_serializer
+        @active_model_serializer ||= "#{self.name}Serializer".safe_constantize
+      end
+    else
+      def active_model_serializer
+        return @active_model_serializer if defined?(@active_model_serializer)
+
+        begin
+          @active_model_serializer = "#{self.name}Serializer".constantize
+        rescue NameError => e
+          raise unless e.message =~ /uninitialized constant/
+        end
       end
     end
   end

test/serializer_support_test.rb

@@ -0,0 +1,11 @@
+require "test_helper"
+
+class RandomModel
+  include ActiveModel::SerializerSupport
+end
+
+class SerializerSupportTest < ActiveModel::TestCase
+  test "it returns nil if no serializer exists" do
+    assert_equal nil, RandomModel.new.active_model_serializer
+  end
+end