title: How to use attrs required dependent on a fields many2one in openerp?
body: <p>I have two class:</p>
  
  <pre><code>class hr_holidays(osv.osv):
      _inherit = 'hr.holidays'
      _columns = {
          'holiday_status_id': fields.many2one("hr.holidays.status", "Loại nghỉ", required=True,readonly=True, states={'draft':[('readonly',False)], 'confirm':[('readonly',False)]}, track_visibility='onchange'),
          'cost': fields.float('Cost', track_visibility='onchange'),
          }
  class hr_holidays_status(osv.osv):
      _name = "hr.holidays.status"
      _columns = {
          'is_cost': fields.boolean('Cost'),
          }
  </code></pre>
  
  <p>Now, I write in file .xml of <code>hr_holidays</code>  with '<code>cost</code>' have attribute <code>required</code> when  <code>is_cost</code> of <code>hr.holidays.status</code> = True</p>
  
  <pre><code>&lt;field name="cost" attrs="{'required':[('is_cost','=',True)]}"/&gt;
  </code></pre>
  
  <p>But Odoo not understand <code>is_cost</code> and error.
  I hope to get your help. Thank you very much. 
  Best Regards</p>