gem "devise"
gem "devise_ldap_authenticatable"

bundle install
rails g devise:install
rails g devise user
rails g devise:views
rails g devise_ldap_authenticatable:install
rails g migration add_username_to_users username:string:index
bundle exec rake db:migrate

before_action :authenticate_user!
before_action :configure_permitted_parameters, if: :devise_controller?

protected

def configure_permitted_parameters
  devise_parameter_sanitizer.for(:sign_in) << :username
end

class User < ActiveRecord::Base
  # Include default devise modules. Others available are:
  # :confirmable, :lockable, :timeoutable and :omniauthable
  devise :ldap_authenticatable, :registerable,
     :recoverable, :rememberable, :trackable, :validatable

  validates :username, presence: true, uniqueness: true

  before_validation :get_ldap_email
  def get_ldap_email
    self.email = Devise::LDAP::Adapter.get_ldap_param(self.username,"mail").first
  end

  # use ldap uid as primary key
  before_validation :get_ldap_id
  def get_ldap_id
    self.id = Devise::LDAP::Adapter.get_ldap_param(self.username,"uidnumber").first
  end

  # hack for remember_token
  def authenticatable_token
    Digest::SHA1.hexdigest(email)[0,29]
  end
end

change email to username

Devise.setup do |config|
  # ==> LDAP Configuration 
  config.ldap_logger = true
  config.ldap_create_user = true
  config.ldap_update_password = true
  config.ldap_use_admin_to_bind = true

  config.authentication_keys = [ :username ]
  config.password_length = 0..128 # if your ldap has a weak password police