gistfile1.rb

# Ability Class
class AdminAbility
  include CanCan::Ability
  
  def initialize(user)

    if user && user.admin?
        can :access, :rails_admin
        can :manage, :all
    end
  end
end

# User Model
class User < ActiveRecord::Base
    attr_accessor :password
    before_save :encrypt_password

    validates_confirmation_of :password
    validates_presence_of :password, :on => :create
    validates_presence_of :email, :on => :create
    validates_uniqueness_of :email

    def user_params
        params.require(:user).permit(:email, :password, :password_confirmation)
    end
    
    def create
        User.create(user_params)
    end
    
    def admin?
        return true
    end

    def self.authenticate(email, password)
        user = find_by_email(email)
        if user && user.password_hash == BCrypt::Engine.hash_secret(password, user.password_salt)
            user
        else
            nil
        end
    end

    def encrypt_password
        if password.present?
            self.password_salt = BCrypt::Engine.generate_salt
            self.password_hash = BCrypt::Engine.hash_secret(password, password_salt)
        end
    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.
  protect_from_forgery with: :exception
  helper_method :current_user
  
  def current_user
      @current_user ||= User.find_by_id(session[:user_id]) if session[:user_id]
  end
  
end

# config/initializers/rails_admin.rb
RailsAdmin.config do |config|

  ### Popular gems integration

  ## == Devise ==
  # config.authenticate_with do
  #  warden.authenticate! scope: :user
  # end
  # config.current_user_method(&:current_user)

  ## == Cancan ==
  config.authorize_with :cancan, AdminAbility

  ## == PaperTrail ==
  # config.audit_with :paper_trail, 'User', 'PaperTrail::Version' # PaperTrail >= 3.0.0

  ### More at https://github.com/sferik/rails_admin/wiki/Base-configuration

  config.actions do
    dashboard                     # mandatory
    index                         # mandatory
    new
    export
    bulk_delete
    show
    edit
    delete
    show_in_app

    ## With an audit adapter, you can add:
    # history_index
    # history_show
  end
end