Rails Devise LDAP Error: NoMethodError in Devise::SessionsController#create - ruby-on-rails

Question

My team has setup my ruby on rails app to work with devise ldap. Whenever a user logs in for the first time, it creates a new entry in the Users model and works completely fine. But whenever a user that already exist in the Users database tries to login it gives the following error:
NoMethodError in Devise::SessionsController#create
undefined method `to' for nil:NilClass
From looking at the server console, it seems like it is failing on this SQL statement:
SELECT `users`.* FROM `users` WHERE `users`.`username` = 'jDoe' LIMIT 1
I think the server is getting no results from that query.
But whenever I run that same sql statement in the ruby console, it gives the correct result (the record of the user attempting to login)
Here are some of my devise configs. I am new to devise and ruby (and I also didn't install the devise) so let me know if you need other config files to debug this problem.
initializers/devise.rb
Devise.setup do |config|
config.ldap_create_user = true
config.ldap_update_password = false
config.mailer_sender = "****"
require 'devise/orm/active_record'
config.authentication_keys = [ :username ]
config.case_insensitive_keys = [ :email ]
config.strip_whitespace_keys = [ :email ]
config.skip_session_storage = [:http_auth]
config.stretches = Rails.env.test? ? 1 : 10
config.reconfirmable = true
config.password_length = 8..128
config.sign_out_via = :delete
end
ldap.yml
authorizations: &AUTHORIZATIONS
group_base: ou=groups,dc=test,dc=com
required_groups:
- cn=admins,ou=groups,dc=test,dc=com
- cn=users,ou=groups,dc=test,dc=com
- ["moreMembers", "cn=users,ou=groups,dc=test,dc=com"]
require_attribute:
objectClass: inetOrgPerson
authorizationRole: postsAdmin
development:
host: ****
port: 636
attribute: uid
base: ou=People,dc=***,dc=com
admin_user: cn=admin,dc=test,dc=com
admin_password: admin_password
ssl: true
test:
host: localhost
port: 3389
attribute: cn
base: ou=people,dc=test,dc=com
admin_user: cn=admin,dc=test,dc=com
admin_password: admin_password
ssl: simple_tls
production:
host: localhost
port: 636
attribute: cn
base: ou=people,dc=test,dc=com
admin_user: cn=admin,dc=test,dc=com
admin_password: admin_password
ssl: start_tls
user.rb
class User < ActiveRecord::Base
devise :ldap_authenticatable, :rememberable, :trackable
attr_accessible :name, :cell, :email, :pref, :type, :username, :password, :password_confirmation, :remember_me
end

Answer 1

My problem happened because I had 2 tables that inherited from User (Client, and Runner) and they both tried to authenticate with the same ldap. When I took out the single table inheritance everything worked fine.