Error Handling - Email LDAP - Rails

I'm beginner in Ruby on Rails and I need help

In my project, I need to catch on LDAP the e-mail of user.

class User < ActiveRecord::Base

 # Include default devise modules. Others available are:
 # :confirmable, :lockable, :timeoutable and :omniauthable
 devise :ldap_authenticatable,:rememberable, :trackable

 #validates :username, presence: true, uniqueness: true
 validates :email, presence: true
 validates_uniqueness_of :email 
 before_validation :get_ldap_email
 before_save :get_ldap_email

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

But If in LDAP doesn't have an e-mail, shows an Rails´ error message and I want to redirect to a page with a message made by me.

So, in SessionController i want make an "if", but i don't have success to call the method.

class Users::SessionsController < Devise::SessionsController
require_relative '../../models/User.rb'

def create
 super
 if get_ldap_email == nil
  # redirect to /errorlogin
 end
end

And here show message

undefined local variable or method `get_ldap_email' for # 
Users::SessionsController:0x67342e0>

Also I modificed the User for this...

def get_ldap_email

 ldapEmail =  Devise::LDAP::Adapter.get_ldap_param(self.username,"mail")
 if ldapEmail == nil

 else
  self.email = ldapEmail.first
 end
end

But now show this message

Validation failed: Email can't be blank

I don't what I need to do...