Makes AR use AMo to_key implementation

[#5249]

activerecord/lib/active_record/attribute_methods/primary_key.rb

@@ -3,12 +3,6 @@ module AttributeMethods
     module PrimaryKey
       extend ActiveSupport::Concern
 
-      # Returns this record's primary key value wrapped in an Array
-      # or nil if the record is a new_record?
-      def to_key
-        new_record? ? nil : [ id ]
-      end
-
       module ClassMethods
         # Defines the primary key field -- can be overridden in subclasses. Overwriting will negate any effect of the
         # primary_key_prefix_type setting, though.

activerecord/test/cases/primary_keys_test.rb

@@ -26,7 +26,7 @@ def test_to_key_with_customized_primary_key
   def test_to_key_with_primary_key_after_destroy
     topic = Topic.find(1)
     topic.destroy
-    assert_equal [1], topic.to_key
+    assert_equal nil, topic.to_key
   end
 
   def test_integer_key