Why does current_user session become nil when updating a user?

Question

I'm using Devise and CanCan for user authentication and administrating roles restricting access to parts of my Rails 4 app for certain users.

I've run into some problems with updating a user. The update works fine and the user object in the db get updated as it should, but my user session is lost on the following redirect_to my user show action. current_user becomes nil which means that CanCan restricts the access to the user show action.

Why does current_user become nil after update, when this does not happen on other actions (e.g create, destroy etc.)?

These are the devise settings in my user model:

devise :database_authenticatable, :registerable,
     :recoverable, :rememberable, :trackable, :validatable, :authentication_keys => [:login]

This is my users_controller.rb's update method:

class UsersController < ApplicationController

  load_and_authorize_resource
  before_filter :authenticate_user!

  def update
    @user = User.find(params[:id])
    if params[:user][:password].blank?
        params[:user].delete(:password)
    end

    respond_to do |format|
      if @user.update_attributes(user_params)
        format.html { redirect_to user_path, :notice => 'User was successfully updated.' }
        format.json { head :ok }
      else
        format.html { render :action => "edit" }
        format.json { render :json => @user.errors, :status => :unprocessable_entity }
      end
    end
  end
end

And this is my ability.rb file:

class Ability
  include CanCan::Ability

  def initialize(user)
    user ||= User.new # guest user (not logged in)
    if defined?(user.role_id)
      if user.role? :admin, user.role_id
        can :manage, :all
      elsif user.role? :hauler, user.role_id
        can :manage, [User,Trip,Invoice], user_id: user.id.to_s
      else
        can :create, :Trip
      end
    end
  end
end

Answer 1

It depends on the update being performed. Sessions are serialized with certain bits of user data.

For instance updating the password will cause a session to be nullified because the encrypted password is part of the serialized hash, and if that is changed, the session can no longer reference the original encrypted password.