Deviseのconfirmable - 確認メールのリンクをクリックしたら即サインインする ref: http://qiita.com/mnishiguchi/items/85df424577326f4207b1

confirmations_controller.rb

# Override
class ConfirmationsController < Devise::ConfirmationsController
  def show
    self.resource = resource_class.confirm_by_token(params[:confirmation_token])
    yield resource if block_given?

    if resource.errors.empty?
      set_flash_message(:notice, :confirmed) if is_flashing_format?

      sign_in(resource) # この一行を加えるのみ

      respond_with_navigational(resource){ redirect_to after_confirmation_path_for(resource_name, resource) }
    else
      respond_with_navigational(resource.errors, :status => :unprocessable_entity){ render :new }
    end
  end
end

routes.rb

Rails.application.routes.draw do
  # ...

  devise_for :users, controllers: { confirmations: 'confirmations' }

  # ...
end