Rails 3: Customized exception handling

Exceptions happen. There’s no way around that. But not all exceptions are created equally.

For instance, a 404 “Not found” error can (and should) be handled correctly in your application.

Let me give you an example of how to handle a ActiveRecord::RecordNotFound exception. Let’s assume you have an application that could show a user profile:

# GET /p/:name
def show
  @profile = Profile.find(params[:name])
end

Now, it may happen that the :name paramater contains a value that cannot be found in our database, most likely because someone made a typo in the URL.

If Profile#find cannot get a proper result it will throw ActiveRecord::RecordNotFound.

Now, instead of showing the user the (by default ugly) 404 page from public/404.html we want to do something more fancy.

Action-specific exception handling

#

Here’s one solution:

# GET /p/:name
def show
  @profile = Profile.find(params[:name])
rescue
  render :template => 'application/profile_not_found', :status => :not_found
end

You can now create app/views/applicaiton/profile_not_found.html.haml and give a nice custom error message to your user.

You may try to find some matching profiles to :name or show a search box.

Global exception handling

#

The above example only works for the specific profile show action. It’s also possible to hanlde exceptions on the application level.

Your show action still looks like this:

# GET /p/:name
def show
  @profile = Profile.find(params[:name])
end

Then, in your app/controllers/application_controller.rb add this:

class ApplicationController < ActionController::Base
  rescue_from ActiveRecord::RecordNotFound, :with => :rescue_not_found

  protected
  def rescue_not_found
    render :template => 'application/not_found', :status => :not_found
  end
end

Whenever an ActiveRecord::RecordNotFound exception is thrown (and not handled by the action itself), it will be handled by your ApplicationController.

Custom exceptions

#

It’s possible to throw your own custom exceptions and handle them in different ways. Like this:

# Define your own error
class MyApp::ProfileNotFoundError < StandardError
end

# GET /p/:name
def show
  @profile = Profile.find_by_name(params[:name])
  raise MyApp::ProfileNotFoundError if @profile.nil?
end

And add this to your ApplicationController:

rescue_from MyApp::ProfileNotFoundError, :with => :profile_not_found

Optionally, if you don’t want to write that custom profile_not_found method, you may also supply a block:

rescue_from MyApp::ProfileNotFoundError do |exception|
  render :nothing => "Profile not found.", :status => 404
end