title: track changes on certain field in odoo 8
body: <p>i am trying record changes done on certain field and display them as messages threads but i can't display messages threads although i have done steps in documentation 
  here is simple code i test on </p>
  
  <p>models.py </p>
  
  <pre class="lang-py prettyprint-override"><code>from openerp import models, fields, api
  
  
  class model_test(models.Model):
  
      _name = "testt.model_test"
      _inherit = ['mail.thread', 'ir.needaction_mixin']
      testt = fields.Char(string="test_text",track_visibility='onchange')
  </code></pre>
  
  <p>views.xml</p>
  
  <pre class="lang-xml prettyprint-override"><code>&lt;?xml version="1.0" encoding="utf-8"?&gt;
  &lt;openerp&gt;
      &lt;data&gt;
  
      &lt;act_window id="test_main_act" name="testt" res_model="testt.model_test" view_mode="tree,form" &gt;
  
      &lt;/act_window&gt;
  
      &lt;menuitem id="test_module_root" name="test module" action="test_main_act"/&gt;
  
          &lt;record id="test_list" model="ir.ui.view"&gt;
              &lt;field name="name"&gt; test form&lt;/field&gt;
              &lt;field name="model"&gt;testt.model_test&lt;/field&gt;
              &lt;field name="arch" type="xml"&gt;
                  &lt;form&gt;
                      &lt;header&gt;
                          &lt;button name="add_one" type="object" string="Add one" /&gt;
  
                      &lt;/header&gt;
                      &lt;sheet&gt;
                          &lt;field name="testt" /&gt;
                      &lt;/sheet&gt;
  
                  &lt;/form&gt;
  
                   &lt;div class="oe_chatter"&gt;
                      &lt;field name="message_follower_ids" widget="mail_followers" groups="base.group_user"/&gt;
                      &lt;field name="message_ids" widget="mail_thread"/&gt;
                  &lt;/div&gt;
              &lt;/field&gt;
  
          &lt;/record&gt;
  
          &lt;record id="test_list" model="ir.ui.view"&gt;
              &lt;field name="name"&gt; test list&lt;/field&gt;
              &lt;field name="model"&gt;testt.model_test&lt;/field&gt;
              &lt;field name="arch" type="xml"&gt;
                  &lt;tree&gt;
                      &lt;field name="testt"&gt;&lt;/field&gt;
                  &lt;/tree&gt;
              &lt;/field&gt;
  
          &lt;/record&gt;
      &lt;/data&gt;
  &lt;/openerp&gt;
  </code></pre>
  
  <p>then the result isn't what i expect 
  <a href="https://i.stack.imgur.com/S073y.png" rel="nofollow noreferrer">unexpected result</a></p>