How to solve NoMethodError with Pundit - ruby-on-rails

Question

I don't know if I'm doing something wrong here but it seems like.
I use Pundit for authorization and I have set up a few models with it now.
Ive got a Category model which can only be created by admins. Also I don't want users to see the show/edit/destroy views either. I just want it to be accessed by admins. So far so good.
Will add some code below:
category_policy.rb
class CategoryPolicy < ApplicationPolicy
def index?
user.admin?
end
def create?
user.admin?
end
def show?
user.admin?
end
def new?
user.admin?
end
def update?
return true if user.admin?
end
def destroy?
return true if user.admin?
end
end
categories_controller.rb
class CategoriesController < ApplicationController
layout 'scaffold'
before_action :set_category, only: %i[show edit update destroy]
# GET /categories
def index
#category = Category.all
authorize #category
end
# GET /categories/1
def show
#category = Category.find(params[:id])
authorize #category
end
# GET /categories/new
def new
#category = Category.new
authorize #category
end
# GET /categories/1/edit
def edit
authorize #category
end
# POST /categories
def create
#category = Category.new(category_params)
authorize #category
if #category.save
redirect_to #category, notice: 'Category was successfully created.'
else
render :new
end
end
# PATCH/PUT /categories/1
def update
authorize #category
if #category.update(category_params)
redirect_to #category, notice: 'Category was successfully updated.'
else
render :edit
end
end
# DELETE /categories/1
def destroy
authorize #category
#category.destroy
redirect_to categories_url, notice: 'Category was successfully destroyed.'
end
private
# Use callbacks to share common setup or constraints between actions.
def set_category
#category = Category.find(params[:id])
end
# Only allow a trusted parameter "white list" through.
def category_params
params.require(:category).permit(:name)
end
end
application_policy.rb
class ApplicationPolicy
attr_reader :user, :record
def initialize(user, record)
#user = user
#record = record
end
def index?
false
end
def create?
create?
end
def new?
create?
end
def update?
false
end
def edit?
update?
end
def destroy?
false
end
class Scope
attr_reader :user, :scope
def initialize(user, scope)
#user = user
#scope = scope
end
def resolve
scope.all
end
end
end
Ive got Pundit included in my ApplicationController and rescue_from Pundit::NotAuthorizedError, with: :forbidden this set up there as well.
The authorization itself works, if I'm logged in with an admin account I have access to /categories/*. If I'm logged out I get the following: NoMethodError at /categories
undefined methodadmin?' for nil:NilClass`
While writing the question I think I found the problem- I guess Pundit looks for a User that is nil because I'm not logged in. What would the correct approach of solving this issue look like?
Best regards

Answer 1

The most common approach is to redirect users from pages that are not accessible by not logged in users. Just add a before action in your controller:
class CategoriesController < ApplicationController
before_action :redirect_if_not_logged_in
<...>
private
def redirect_if_not_logged_in
redirect_to :home unless current_user
end
end
(I assume here that you have current_user method which returns user instance or nil. Please change :home to wherever you want to redirect users)

Answer 2

There are multiple ways of achieving what you want.
The most obvious (but kind of dirty) and straightforward-looking way of doing that would be to add a check for user presence in every condition:
user && user.admin?
It won't fail with nil error as the second part of the condition won't get executed. But it doesn't look very nice, right? Especially if you have to copy this over to all methods you have in CategoryPolicy.
What you can do instead, is to make Pundit "think" that you passed a User, by creating a GuestUser class which responds with false to admin? method (https://en.wikipedia.org/wiki/Null_object_pattern):
In object-oriented computer programming, a null object is an object with no referenced value or with defined neutral ("null") behavior. The null object design pattern describes the uses of such objects and their behavior (or lack thereof)
And use it when user is nil. In practice, it will look like this:
class ApplicationPolicy
attr_reader :user, :record
def initialize(user, record)
#user = user || GuestUser.new
#record = record
end
# ...
end
class GuestUser
def admin?
false
end
end
This way you won't have to alter any of your other code, as the model you passed responds to the method which is expected by policy (admin?). You may want to define this GuestUser somewhere else (not in the policy file), depending if you want other parts of the app to reuse that behavior.
You can also proceed with the redirect approach from P. Boro answer. It's less flexible in some sense but can totally work fine if you don't need anything besides redirecting all non-logged in users.