Rescue exceptions raised in controller actions.

rescue_from receives a series of exception classes or class names, and a trailing :with option with the name of a method or a Proc object to be called to handle them. Alternatively a block can be given.

Handlers that take one argument will be called with the exception, so that the exception can be inspected when dealing with it.

Handlers are inherited. They are searched from right to left, from bottom to top, and up the hierarchy. The handler of the first class for which exception.is_a?(klass) holds true is the one invoked, if any.

class ApplicationController < ActionController::Base

  rescue_from User::NotAuthorized, :with => :deny_access # self defined exception
  rescue_from ActiveRecord::RecordInvalid, :with => :show_errors

  rescue_from 'MyAppError::Base' do |exception|
    render :xml => exception, :status => 500
  end

  protected
    def deny_access
      ...
    end

    def show_errors(exception)
      exception.record.new_record? ? ...
    end

end

[Source]

     # File vendor/rails/actionpack/lib/action_controller/rescue.rb, line 88
 88:       def rescue_from(*klasses, &block)
 89:         options = klasses.extract_options!
 90:         unless options.has_key?(:with)
 91:           block_given? ? options[:with] = block : raise(ArgumentError, "Need a handler. Supply an options hash that has a :with key as the last argument.")
 92:         end
 93: 
 94:         klasses.each do |klass|
 95:           key = if klass.is_a?(Class) && klass <= Exception
 96:             klass.name
 97:           elsif klass.is_a?(String)
 98:             klass
 99:           else
100:             raise(ArgumentError, "#{klass} is neither an Exception nor a String")
101:           end
102: 
103:           # Order is important, we put the pair at the end. When dealing with an
104:           # exception we will follow the documented order going from right to left.
105:           rescue_handlers << [key, options[:with]]
106:         end
107:       end