For an application we need some authorization method, Policies in rails is one of them.
There is one library Pundit used for authorization.
Pundit gem uses object oriented design and Ruby classes.

To start with add this gems into your Gemfile and run bundle install:

gem "pundit"

Another method to set up an application policy using generator is :

rails g pundit:install

It will create a app/policies/ directory. After this restart the server so that it will pick up classes in the pp/policies/ directory.

Ones you have generated your application policy after that restart the Rails server so that Rails can pick up any classes in the app/policies directory.

Now create a policy class in app/policies/post_policy.rb

class PostPolicy < ApplicationPolicy
  def destroy?
    user.admin?
  end
end

Inside the controller we need to check our rule:

class PostController < ApplicationController

    def destroy
      authorize @post
      @post.destroy

      redirect_to posts_url, notice: 'Post was successfully destroyed.'
    end
end