# File activesupport/lib/active_support/rescuable.rb, line 51
def rescue_from(*klasses, &block)
  options = klasses.extract_options!

  unless options.has_key?(:with)
    if block_given?
      options[:with] = block
    else
      raise ArgumentError, "Need a handler. Supply an options hash that has a :with key as the last argument."
    end
  end

  klasses.each do |klass|
    key = if klass.is_a?(Class) && klass <= Exception
      klass.name
    elsif klass.is_a?(String)
      klass
    else
      raise ArgumentError, "#{klass} is neither an Exception nor a String"
    end

    # put the new handler at the end because the list is read in reverse
    self.rescue_handlers += [[key, options[:with]]]
  end
end

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

Exceptions raised inside exception handlers are not propagated up.