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The CanCan::AccessDenied exception is raised when calling authorize! in the controller and the user is not able to perform the given action. A message can optionally be provided.

authorize! :read, Article, :message => "Unable to read this article."

This exception can also be raised manually if you want more custom behavior.

raise CanCan::AccessDenied.new("Not authorized!", :read, Article)

The message can also be customized through internationalization.

# in config/locales/en.yml
en:
  unauthorized:
    manage:
      all: "Not authorized to %{action} %{subject}."
      user: "Not allowed to manage other user accounts."
    update:
      project: "Not allowed to update this project."

Notice manage and all can be used to generalize the subject and actions. Also %{action} and %{subject} can be used as variables in the message.

You can catch the exception and modify its behavior in the ApplicationController. For example here we set the error message to a flash and redirect to the home page.

class ApplicationController < ActionController::Base
  rescue_from CanCan::AccessDenied do |exception|
    redirect_to main_app.root_url, :alert => exception.message
  end
end

The action and subject can be retrieved through the exception to customize the behavior further.

exception.action # => :read
exception.subject # => Article

The default error message can also be customized through the exception. This will be used if no message was provided.

exception.default_message = "Default error message"
exception.message # => "Default error message"

If you prefer to return the 403 Forbidden HTTP code, create a public/403.html file and write a rescue_from statement like this example in ApplicationController:

class ApplicationController < ActionController::Base
  rescue_from CanCan::AccessDenied do |exception|
    render :file => "#{Rails.root}/public/403.html", :status => 403, :layout => false
    ## to avoid deprecation warnings with Rails 3.2.x (and incidentally using Ruby 1.9.3 hash syntax)
    ## this render call should be:
    # render file: "#{Rails.root}/public/403", formats: [:html], status: 403, layout: false
  end
end 

403.html must be pure HTML, CSS, and JavaScript--not a template. The fields of the exception are not available to it.

If you are getting unexpected behavior when rescuing from the exception it is best to add some logging . See Debugging Abilities for details.

See Authorization in Web Services for rescuing exceptions for XML responses.

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