We can't just piggyback on serialization because it may already be included at this point.

lib/active_model_serializers.rb

@@ -1,7 +1,7 @@
 require "active_model"
 require "active_model/serializer"
 
-ActiveModel::Serialization.class_eval do
+module ActiveModel::SerializerSupport
   extend ActiveSupport::Concern
 
   module ClassMethods #:nodoc:
@@ -25,6 +25,10 @@ ActiveModel::Serialization.class_eval do
   alias :read_attribute_for_serialization :send
 end
 
+ActiveSupport.on_load(:active_record) do
+  include ActiveModel::SerializerSupport
+end
+
 begin
   require 'action_controller'
   require 'action_controller/serialization'

test/serializer_test.rb

@@ -16,7 +16,7 @@ class SerializerTest < ActiveModel::TestCase
   end
 
   class User
-    include ActiveModel::Serialization
+    include ActiveModel::SerializerSupport
 
     attr_accessor :superuser