Not able to find admin? while doing Michael Hartl's Tutorial - ruby-on-rails

Question

This is my users controller and i am doing michael hartl's tutorial but i am having problem in current_user.admin? in the admin_user method defined in users controller and also having problem in my destroy method as i am not able to delete user as well.
pls any solution??
class UsersController < ApplicationController
before_action :logged_in_user, only: [:index, :edit, :update, :destroy]
before_action :correct_user, only: [:edit, :update]
before_action :admin_user, only: :destroy
def index
#users = User.paginate(page: params[:page])
end
def new
#user = User.new
end
def show
#user = User.find(params[:id])
end
def create
#user = User.new(user_params)
if #user.save
flash[:success] = 'Welcome to the Sample App!'
redirect_to #user
else
render 'new'
end
end
def edit
#user = User.find(params[:id])
end
def update
#user = User.find(params[:id])
if #user.update_attributes(user_params)
flash[:success] = 'Profile Updated'
redirect_to #user
else
render 'edit'
end
end
def destroy
User.find(params[:id]).destroy
flash[:success] = 'User deleted'
redirect_to users_url
end
private
def user_params
params.require(:user).permit(:name, :email, :password,
:password_confirmation)
end
def logged_in_user
unless logged_in?
store_location
flash[:danger] = 'Please log in.'
redirect_to login_url
end
end
def correct_user
#user = User.find(params[:id])
redirect_to(root_url) unless current_user?(#user)
end
def admin_user
redirect_to(root_url) unless current_user.admin?
end
end
User.rb
class User < ActiveRecord::Base
attr_accessor :remember_token
before_save { self.email = email.downcase }
validates :name, presence: true, length: { maximum: 50 }
VALID_EMAIL_REGEX = /\A[\w+\-.]+#[a-z\d\-.]+\.[a-z]+\z/i
validates :email, presence: true, length: { maximum: 255 },
format: { with: VALID_EMAIL_REGEX },
uniqueness: true
has_secure_password
validates :password, length: { minimum: 6 }
def User.digest(string)
cost = ActiveModel::SecurePassword.min_cost ? BCrypt::Engine::MIN_COST :
BCrypt::Engine.cost
BCrypt::Password.create(string, cost: cost)
end
def User.new_token
SecureRandom.urlsafe_base64
end
def remember
self.remember_token = User.new_token
update_attribute(:remember_digest, User.digest(remember_token))
end
def authenticated?(remember_token)
return false if remember_digest.nil?
BCrypt::Password.new(remember_digest).is_password?(remember_token)
end
def forget
update_attribute(:remember_digest, nil)
end
end
Link for the Delete
<li>
<%= gravatar_for user %>
<%= link_to user.name, user %>
<% if current_user.admin? && !current_user?(user) %>
| <%= link_to 'delete', user, method: :delete,
data: { confirm: 'You sure?' } %>
<% end %>
</li>