ControllerExceptions are a way of simplifying controller code by placing exceptional logic back into the MVC pattern.

When a ControllerException is raised within your application merb will attempt to re-route the request to your Exceptions controller to render the error in a friendly mannor.

For example you might have an action in your app that raises NotFound if a some resource was not available

def show
  product = Product.find(params[:id])
  raise NotFound if product.nil?
  [...]
end

This would halt execution of your action and re-route it over to your Exceptions controller which might look something like

class Exceptions < Application

def not_found
  render :layout => :none
end

end

As usual the not_found action will look for a template in

app/views/exceptions/not_found.html.erb

Note: All standard ControllerExceptions have an HTTP status code associated with them which is sent to the browser when the action it is rendered.

Note: If you do not specifiy how to handle raised ControllerExceptions or an unhandlable exception occurs within your customised exception action then they will be rendered using the built-in error template in development mode this "built in" template will show stack-traces for any of the ServerError family of exceptions (you can force the stack-trace to display in production mode using the :exception_details config option in merb.yml)

Internal Exceptions

Any other rogue errors (not ControllerExceptions) that occur during the execution of you app will be converted into the ControllerException InternalServerError, and like all ControllerExceptions can be caught on your Exceptions controller.

InternalServerErrors return status 500, a common use for cusomizing this action might be to send emails to the development team, warning that their application have exploded. Mock example:

def internal_server_error
  MySpecialMailer.deliver(
    "team@cowboys.com",
    "Exception occured at #{Time.now}",
    params[:exception])
  render :inline => 'Something is wrong, but the team are on it!'
end

Note: The special param[:exception] is available in all Exception actions and contains the ControllerException that was raised (this is handy if you want to display the associated message or display more detailed info)

Extending ControllerExceptions

To extend the use of the ControllerExceptions one may extend any of the HTTPError classes.

As an example we can create an exception called AdminAccessRequired.

class AdminAccessRequired < Merb::ControllerExceptions::Unauthorized; end

Add the required action to our Exceptions controller

class Exceptions < Application
  def admin_access_required
    render
  end
end

In app/views/exceptions/admin_access_required.rhtml

<h1>You're not an administrator!</h1>
<p>You tried to access <%= @tried_to_access %> but that URL is
restricted to administrators.</p>