add ActiveAdmin

app/models/admin_user.rb

@@ -0,0 +1,18 @@
+class AdminUser < ActiveRecord::Base
+  # Include default devise modules. Others available are:
+  # :token_authenticatable, :encryptable, :confirmable, :lockable, :timeoutable and :omniauthable
+  devise :database_authenticatable,
+         :recoverable, :rememberable, :trackable, :validatable
+
+  attr_accessor :login
+
+  protected
+
+    def self.find_for_database_authentication(warden_conditions)
+      conditions = warden_conditions.dup
+      login = conditions.delete(:login)
+      where(conditions).where(["lower(email) = :value", { :value => login.strip.downcase }]).first
+    end
+
+end
+