title: How to check if one2many field is empty in openerp xml views
body: <p>I'm trying to make some field invisible if another field ( which is one2many ) has no value ( is empty).</p>
  
  <p>I'm trying something like</p>
  
  <pre><code>&lt;field name="reference" invisible="{'line_ids', '=', False}"/&gt;
  </code></pre>
  
  <p>Also tried</p>
  
  <pre><code>&lt;field name="reference" invisible="{'line_ids', 'in', []}"/&gt;
  </code></pre>
  
  <p>And finally</p>
  
  <pre><code>&lt;field name="reference" invisible="{'line_ids', '=', None}"/&gt;
  </code></pre>
  
  <p><em>Note: line_ids is one2many field</em></p>
  
  <p>But did not work. Somebody please suggest if some possible way to do this.</p>