ruby on rails - Pundit policy_scope error: undefined method `admin?' for nil:NilClass

Question

Running into something I don't understand with Pundit,

Using Rails 4.2.5.1, Pundit 1.1.0 with Devise for authentication.

I'm trying to use a policy scope for the BlogController#Index action.

• If user is admin, display all posts (drafts, published)
• If user is standard, display posts marked published only
• If no user / user not logged in, display posts marked published only

Getting an error:

undefined method `admin?' for nil:NilClass

Live shell reveals:

>> user
=> nil

---

# ApplicationController
class ApplicationController < ActionController::Base
  include Pundit
  rescue_from Pundit::NotAuthorizedError, with: :user_not_authorized

  # Prevent CSRF attacks by raising an exception.
  # For APIs, you may want to use :null_session instead.
  protect_from_forgery with: :exception

  private

    def user_not_authorized
      flash[:error] = "You are not authorized to perform this action."
      redirect_to(request.referrer || root_path)
    end
end

---

# BlogController

# == Schema Information
#
# Table name: blogs
#
#  id         :integer          not null, primary key
#  title      :string           default(""), not null
#  body       :text             default(""), not null
#  published  :boolean          default("false"), not null
#  created_at :datetime         not null
#  updated_at :datetime         not null
#

class BlogsController < ApplicationController
  before_action :set_blog, only: [:show, :edit, :update, :destroy]
  before_action :authenticate_user!, except: [:index, :show]

  after_action :verify_authorized, except: [:index, :show]
  after_action :verify_policy_scoped, only: [:index]

  def index
    @blogs = policy_scope(Blog)
    authorize @blog
  end

  def show
  end

  def new
    @blog = Blog.new
    authorize @blog
  end

  def edit
    authorize @blog
  end

  def create
    @blog = Blog.new(blog_params)
    @blog.user = current_user if user_signed_in?

    authorize @blog

    if @blog.save
      redirect_to @blog, notice: "Blog post created."
    else
      render :new
    end
  end

  def update
    authorize @blog

    if @blog.update(blog_params)
      redirect_to @blog, notice: "Blog updated."
    else
      render :edit
    end
  end

  def destroy
    authorize @blog
    @blog.destroy
    redirect_to blogs_url, notice: "Blog post deleted."
  end

  private

    def set_blog
      @blog = Blog.friendly.find(params[:id])
    end

    def blog_params
      params.require(:blog).permit(*policy(@blog|| Blog).permitted_attributes)
    end
end

---

# Application Policy

class ApplicationPolicy
  attr_reader :user, :record

  def initialize(user, record)
    @user = user
    @record = record
  end

  def index?
    false
  end

  def show?
    scope.where(:id => record.id).exists?
  end

  def create?
    false
  end

  def new?
    create?
  end

  def update?
    false
  end

  def edit?
    update?
  end

  def destroy?
    false
  end

  def scope
    Pundit.policy_scope!(user, record.class)
  end

  class Scope
    attr_reader :user, :scope

    def initialize(user, scope)
      @user = user
      @scope = scope
    end

    def resolve
      scope
    end
  end
end

---

# Blog Policy

class BlogPolicy < ApplicationPolicy
  class  Scope < Scope
    def resolve
      if user.admin?
        scope.all
      else
        scope.where(published: true)
      end
    end
  end

  def new?
    user.admin?
  end

  def index?
    true
  end

  def update?
    user.admin?
  end

  def create?
    user.admin?
  end

  def destroy?
    user.admin?
  end

  def permitted_attributes
    if user.admin?
        [:title, :body]
    end
  end
end

---

In the Pundit BlogPolicy scope I've created:

  class  Scope < Scope
    def resolve
      if user.admin?
        scope.order('id DESC')
      else
        scope.where('published: true')
      end
    end
  end

If I log in as an admin user it works fine.

I'm able to view all blog posts.

If I log in as a standard user it works.

Standard user sees blog posts that are marked published.

If I'm not logged in where user is nil I get an error:

NoMethodError at /blog
undefined method `admin?' for nil:NilClass

I can add another clause elsif user.nil? before user.admin? or a case when statement but I thought if the user is not an admin it should just display what is in the else block?

 # This seems wrong?

  class  Scope < Scope
    def resolve
      if user.nil?
        scope.where('published: true')
      elsif user.admin?
        scope.all
      else
        scope.where('published: true')
      end
    end
  end

Any pointers much appreciated

Answer 1

You can use try:

if user.try(:admin?)
  # do something
end

http://api.rubyonrails.org/v4.2.5/classes/Object.html#method-i-try