Rails: How do I resolve: Pundit::NotDefinedError in PostsController#show? - ruby-on-rails

Question

whenever I run the below program and try to view my posts (in my show view) as any user, I am introduced to this error page:
Pundit::NotDefinedError in PostsController#show
unable to find policy of nil
Within that error page:
def show
#post = Post.find(params[:id])
authorize #posts # <- The error highlights this line
end
I'm not sure how to get around this dilemma as I'm just learning about Pundit Policy rules and am new to Rails and Ruby. Any help would be much appreciated. Below are my policy pages and related pages:
User.rb Model
class User < ActiveRecord::Base
# Include default devise modules. Others available are:
# :confirmable, :lockable, :timeoutable and :omniauthable
devise :database_authenticatable, :registerable,
:recoverable, :rememberable, :trackable, :validatable, :confirmable
has_many :posts
def admin?
role == 'admin'
end
def moderator?
role == 'moderator'
end
def member?
role == 'member'
end
def guest?
role == 'guest'
end
end
Application Controller
class ApplicationController < ActionController::Base
# Prevent CSRF attacks by raising an exception.
# For APIs, you may want to use :null_session instead.
include Pundit
protect_from_forgery with: :exception
before_action :configure_permitted_parameters, if: :devise_controller?
rescue_from Pundit::NotAuthorizedError do |exception|
redirect_to root_url, alert: exception.message
end
protected
def configure_permitted_parameters
devise_parameter_sanitizer.for(:sign_up) << :name
end
end
Posts Controller
class PostsController < ApplicationController
# before_action :flash_attack
# protected
# def flash_attack
# flash[:notice] = "Create/Edit/Comment on a post!"
# end
def index
#posts = Post.all
authorize #posts
end
def show
#post = Post.find(params[:id])
authorize #posts
end
def new
#post = Post.new
authorize #post
end
def create
#post = current_user.posts.build(params.require(:post).permit(:title, :body))
authorize #post
if #post.save
flash[:notice] = "Post was saved."
redirect_to #post
else
flash[:error] = "There was an error saving the post. Please try again."
render :new
end
end
def edit
#post = Post.find(params[:id])
authorize #post
end
def update
#post = Post.find(params[:id])
authorize #post
if #post.update_attributes(params.require(:post).permit(:title, :body))
flash[:notice] = "Post was updated."
redirect_to #post
else
flash[:error] = "There was an error saving the post. Please try again."
render :edit
end
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?
user.present?
end
def new?
create?
end
def update?
user.present? && (record.user == user || user.admin?)
end
def edit?
update?
end
def destroy?
update?
end
def scope
record.class
end
class Scope
attr_reader :user, :scope
def initialize(user, scope)
#user = user
#scope = scope
end
def resolve
scope
end
end
end
Posts Policy
class PostPolicy < ApplicationPolicy
class Scope < Scope
def resolve
if user.admin?
scope.all
else
scope.where(:published => true)
end
end
def index?
true
end
def show?
true
end
def update?
user.admin? or not post.published?
end
end
end
Index View
<h1>All Posts</h1>
<% if policy(Post.new).create? %>
<%= link_to "New Post", new_post_path, class: 'btn btn-success' %>
<% end %>
<% #posts.each do |post| %>
<div class="media">
<div class="media-body">
<h4 class="media-heading">
<%= link_to post.title, post %>
</h4>
<small>
submitted <%= time_ago_in_words(post.created_at) %> ago by <%= post.user.name unless post.user.nil? %><br>
<%= post.comments.count %> Comments
</small>
</div>
</div>
<% end %>
Show View
<h1> <%= #post.title %> </h1>
<% if policy(#post).edit? %>
<%= link_to "Edit", edit_post_path(#post), class: 'btn btn-success' %>
<% end %>
<p> <%= #post.body %> </p>
Thanks in advance everyone. Let me know if any more information would be great.

Answer 1

#posts is nil in show action, you should use #post as such:
authorize #post

Answer 2

I had this issue when working on a Rails 6 API only application with the Pundit gem.
I was running into the error below when I test my Pundit authorization for my controller actions:
Pundit::NotDefinedError - unable to find policy of nil
Here's how I solved:
The instance variables called by the authorize method in your controller must correspond to the instance variable of the controller action being called.
So for the index action it should be #posts:
authorize #posts
For the show action it should be #post:
authorize #post
and for the create action it should be #post
authorize #post
and so on.
That's all.
I hope this helps