User.activeadmin.rb

#ActiveAdmin User 
ActiveAdmin.register User do
  index do
    column :email
    column :alias
    column :current_sign_in_at
    column :last_sign_in_at
    column :sign_in_count
    default_actions
  end

  filter :email


  show do
    h3 user.alias

    attributes_table do
      row :email
      row :first_name
      row :last_name
      row :alias
      row :bio
    end
    table_for  user.roles do
      column "Role" do |role|
        role.name
      end

    end

  end

  form do |f|

    #Debugging
    if f.object.errors.size >= 1
      f.inputs "Errors" do
        f.object.errors.full_messages.join('|')
      end
    end

    f.inputs "Admin Details" do
      f.input :email
      f.input :first_name
      f.input :last_name
      f.input :alias
      f.input :bio
      f.input :password, :hint => "Not Required if not changing the password"  , :required => false
      f.input :password_confirmation
      f.input :roles, :as => :check_boxes
      f.input :photo_link
    end

    f.actions
  end

  controller do
    def permitted_params
      params.permit(:user => [:email,:first_name, :last_name, :alias,:bio, :password, :password_confirmation, :photo_link, :roles, :role_id, :role_ids])
      params.permit(:user_roles => [:user_id, :role_id])
    end

  end


end

user.model.rb

#User Model
class User < ActiveRecord::Base

  rolify
  # Include default devise modules. Others available are:
  # :confirmable, :lockable, :timeoutable and :omniauthable
  if Rails.env.production?
    devise :database_authenticatable, :recoverable, :rememberable, :trackable, :validatable
  else
    devise :database_authenticatable, :recoverable, :rememberable, :trackable, :validatable, :registerable
  end

  validates_presence_of :first_name, :last_name, :alias, :email, :case_sensitive => true
  validates_uniqueness_of :email, :case_sensitive => false
  validates_uniqueness_of :alias, :case_sensitive => false

  has_many :entries

  belongs_to :role

  before_create :assign_role

  def assign_role
    # assign a default role if no role is assigned
    self.add_role :user if self.roles.first.nil?
  end



=begin
  after_create { |admin| admin.send_reset_password_instructions }

  def password_required?
    new_record? ? false : super
  end
=end

end