title: Handling more than one admin types login with ActiveAdmin
body: <p>I have an application requiring three User types(User, Agent, Admin) with login and requiring different devise modules.</p>
  
  <pre><code>                        User    AgentAdmin  AdminUser
  
  confirmable              Yes        Yes         No
  lockable                 Yes        Yes         No
  timeoutable               No        Yes         Yes
  Omniauthable             Yes        No          No
  database_authenticatable Yes        Yes         Yes
  recoverable              Yes        Yes         No
  rememberable             Yes        Yes         No
  trackable                Yes        Yes         Yes
  validatable              Yes        Yes         Yes
  </code></pre>
  
  <p>Further these users may have additional columns based on their type. So, I want to create different tables based on their type.</p>
  
  <p>Depending on my requirements, I decided to handle <code>AgentAdmin</code> and <code>AdminUser</code> with <a href="https://github.com/activeadmin/activeadmin" rel="nofollow noreferrer">ActiveAdmin</a> with different namespaces(admin for <code>AdminUser</code> , agent for <code>AgentAdmin</code>). I have configured <code>authentication_method</code>, <code>current_user_method</code> and <code>logout_link_path</code> for both these namespaces as below.</p>
  
  <pre><code>config.load_paths = [File.join(Rails.root,'app','admin'), File.join(Rails.root,'app','agent')]
  
  config.namespace :admin do |admin|
    admin.authentication_method = :authenticate_admin_user!
    admin.current_user_method = :current_admin_user
    admin.logout_link_path = :destroy_admin_user_session_path
  end
  
  config.namespace :agent do |agent|
    agent.authentication_method = :authenticate_agent_admin!
    agent.current_user_method = :current_agent_admin
    agent.logout_link_path = :destroy_agent_admin_session_path
  end
  </code></pre>
  
  <p>I have the following code in my <code>routes.rb</code>:</p>
  
  <pre><code>devise_for :agent_admins, ActiveAdmin::Devise.config
  devise_for :admin_users, ActiveAdmin::Devise.config
  ActiveAdmin.routes(self)
  </code></pre>
  
  <p>This generates login paths only for <code>admin_users</code>.
  If I set <code>config.default_namespace = :agent</code> in active_admin.rb config file, the login path for <code>agent_admin</code> is generated, but not <code>admin_users</code>. </p>
  
  <p>All other ActiveAdmin paths are generated normally for both the namespaces.</p>
  
  <p>How can I solve, to generate both the <code>login</code> paths handled by <code>activedmin</code>?</p>