title: How to use related fields (fields.related) in odoo-8?
body: <p>I am trying to retrieve comment field(customer internal notes) from res_partner to account invoice module.Right now I just want to print it later I will include it in xml code. I tried in three ways like this,</p>
  
  <pre><code>1)comment2 = fields.Char(string='Comment',related='res_partner.comment',compute='_compute_com')
  @api.multi
  def _compute_com(self):
      print self.comment2
  
  2)comment = fields.Many2one('res.partner','Comment',compute='_compute_com')
    @api.multi
    def _compute_com(self):
      print self.comment
  
  3)partner_comment = fields.Char(compute='_compute_com')
   @api.multi
   def _compute_com(self):
      Comment = self.env['res.partner'].browse(partner_id).comment
      print Comment
  </code></pre>