fix for ruby 1.9.

lib/active_model/serializable.rb

@@ -1,5 +1,11 @@
+require 'active_model/serializable/utils'
+
 module ActiveModel
   module Serializable
+    def self.included(base)
+      base.extend Utils
+    end
+
     def as_json(options={})
       instrument('!serialize') do
         if root = options.fetch(:root, json_key)
@@ -26,14 +32,8 @@ module ActiveModel
       end
     end
 
-    if RUBY_VERSION >= '2.0'
-      def namespace
-        get_namespace && Object.const_get(get_namespace)
-      end
-    else
-      def namespace
-        get_namespace && get_namespace.safe_constantize
-      end
+    def namespace
+      get_namespace && Utils._const_get(get_namespace)
     end
 
     def embedded_in_root_associations

lib/active_model/serializable/utils.rb

@@ -0,0 +1,12 @@
+module ActiveModel
+  module Serializable
+    module Utils
+      extend self
+
+      def _const_get(const)
+        method = RUBY_VERSION >= '2.0' ? :const_get : :qualified_const_get
+        Object.send method, const
+      end
+    end
+  end
+end

lib/active_model/serializer.rb

@@ -55,32 +55,18 @@ end
       end
       attr_reader :key_format
 
-      if RUBY_VERSION >= '2.0'
-        def serializer_for(resource, options = {})
-          if resource.respond_to?(:to_ary)
-            if Object.constants.include?(:ArraySerializer)
-              ::ArraySerializer
-            else
-              ArraySerializer
-            end
+      def serializer_for(resource, options = {})
+        if resource.respond_to?(:to_ary)
+          if Object.constants.include?(:ArraySerializer)
+            ::ArraySerializer
           else
-            begin
-              Object.const_get build_serializer_class(resource, options)
-            rescue NameError
-              nil
-            end
+            ArraySerializer
           end
-        end
-      else
-        def serializer_for(resource, options = {})
-          if resource.respond_to?(:to_ary)
-            if Object.constants.include?(:ArraySerializer)
-              ::ArraySerializer
-            else
-              ArraySerializer
-            end
-          else
-            build_serializer_class(resource, options).safe_constantize
+        else
+          begin
+            _const_get build_serializer_class(resource, options)
+          rescue NameError
+            nil
           end
         end
       end