Let’s jump into the code.

# users_controller.rb

def show
  @user = User.find(params[:id])

  respond_to do |format|
    format.html # show.html.erb
    format.xml  { render :xml => @user }
  end
end

The above method would fine if everything goes fine. However if a request is made for a user id that does not exist then User.find will throw an exception.

If the request was made for an html page then rails will handle the exception and will show the appropriate error page depending on if you are running in development or production mode.

However for .xml there is an issue. If it is an API request then ,in the case of an error, you still need to send an xml response with the error message. Question is how to handle exception in a generic way.

rescue_from is the solution

If you don’t know what rescue_from is then read this blog about rescue_from .

rescue_from Exception, :with => :all_exception_catch
def all_exception_catch(e)
  if params[:format] == 'xml'
     render :text => '<xml><error>There was an error processing the request</error></xml>' 
  else
    rescue_action_without_handler(e)
  end
end

In the above code, you are asking Rails to catch all the exception with the method all_exception_catch. In that method if the format is xml then an xml response is sent. If the format is not xml then you would like the request to travel the path it was travelling. This is important because a lot of plugins like hoptoad latches onto method rescue_action_in_public. You don’t want the exception propogation to halt with method all_exception_catch .