== Parameter Filtering == Rails keeps a log file for each environment (development, test and production) in the "log" folder. These are extremely useful when debugging what's actually going on in your application, but in a live application you may not want every bit of information to be stored in the log file. The `filter_parameter_logging` method can be used to filter out sensitive information from the log. It works by replacing certain values in the `params` hash with "[FILTERED]" as they are written to the log. As an example, let's see how to filter all parameters with keys that include "password": [source, ruby] ------------------------- class ApplicationController < ActionController::Base filter_parameter_logging :password end ------------------------- The method works recursively through all levels of the params hash and takes an optional second parameter which is used as the replacement string if present. It can also take a block which receives each key in return and replaces those for which the block returns true.