diff --git a/activerecord/lib/active_record/attribute_methods.rb b/activerecord/lib/active_record/attribute_methods.rb
index 7fd7accc6b14c5a9f580899f4f514cc22edc7021..8bd51dc71fef31e3c175fc708a9a8bd02b80b791 100644
--- a/activerecord/lib/active_record/attribute_methods.rb
+++ b/activerecord/lib/active_record/attribute_methods.rb
@@ -359,6 +359,8 @@ def column_for_attribute(name)
     # "2004-12-12" in a date column is cast to a date object, like Date.new(2004, 12, 12)). It raises
     # <tt>ActiveModel::MissingAttributeError</tt> if the identified attribute is missing.
     #
+    # Note: +:id+ is always present.
+    #
     # Alias for the <tt>read_attribute</tt> method.
     #
     #   class Person < ActiveRecord::Base