[ci skip] belongs_to in self join association needs optional: true, if it's over 5.0 ver of rails

guides/source/association_basics.md

@@ -505,7 +505,7 @@ class Employee < ApplicationRecord
   has_many :subordinates, class_name: "Employee",
                           foreign_key: "manager_id"
 
-  belongs_to :manager, class_name: "Employee"
+  belongs_to :manager, class_name: "Employee", optional: true
 end
 ```