To implement captcha in RubyonRails, validates_captcha plugin can be a good option but a small customization i need with this plugin was to use it on some specific action and not to be validated the captcha field every time an instance of the model is saved or updated.
Here is a small work-around for its customization…
How [...]

Validates captchais a good pluging to implement captcha in your rails application.
However i found that there is repetition of the string of the image and the quality of image is not that good. To get a good quality image and random string replace the code of the file /vendor/plugins/validates_captcha/lib/captcha_challenge.rb with the following code…

require ‘digest/sha1′
module AngryMidgetPluginsInc [...]