checks if method is defined before defining it

lib/extensions/has_one.rb

@@ -9,9 +9,11 @@ if defined?(::ActiveRecord)
       super
       name = reflection.name
       mixin.class_eval <<-CODE, __FILE__, __LINE__ + 1
+        unless defined? #{name}_id
           def #{name}_id
             association(:#{name}).reader.try(:id)
           end
+        end
       CODE
     end
   end