title: rails activeadmin ask to show a input field if new
body: <p>I have a form in <a href="http://activeadmin.info" rel="nofollow">ActiveAdmin</a>:</p>
  
  <pre><code>form do |f|
    f.inputs t('out_payment_details') do
      f.input :amount
      f.input :status
      f.input :project
      f.input :approved_user
      f.input :request_user
      f.input :from_company
      f.input :to_company
    end
    f.actions
  end
  </code></pre>
  
  <p>What I want:</p>
  
  <p>When new/create, <code>request_user</code> is always the <code>current_user</code>, and if edit(not create), then the <code>approved_user</code> is <code>current_user</code>.</p>
  
  <p>How can I have some conditions of default value with sth like "on new form"?</p>