title: Boolean on_change event error Odoo 8
body: <p>I have an on_change function. The purpose of this on_change is to see when the boolean was last changed. (bool=boolean, datenow=datetime):  </p>
  
  <pre><code>@api.onchange('bool')
  def onchange_bool(self):
      self.datenow=datetime.datetime.now()
  </code></pre>
  
  <p>When I call it(Press the boolean button) in my <code>res.partner</code> I get an error: <code>ValueError: "name 'partner_id' is not defined" while evaluating '[partner_id]'</code> Any ideas/pointers?</p>
  
  <p>EDIT the full error:</p>
  
  <pre><code>Traceback (most recent call last):
      File "/opt/odoo/openerp/http.py", line 537, in _handle_exception
      return super(JsonRequest, self)._handle_exception(exception)
    File "/opt/odoo/openerp/http.py", line 574, in dispatch
      result = self._call_function(**self.params)
    File "/opt/odoo/openerp/http.py", line 310, in _call_function
      return checked_call(self.db, *args, **kwargs)
    File "/opt/odoo/openerp/service/model.py", line 118, in wrapper
      return f(dbname, *args, **kwargs)
    File "/opt/odoo/openerp/http.py", line 307, in checked_call
      return self.endpoint(*a, **kw)
    File "/opt/odoo/openerp/http.py", line 803, in __call__
      return self.method(*args, **kw)
    File "/opt/odoo/openerp/http.py", line 403, in response_wrap
      response = f(*args, **kw)
    File "/opt/odoo/addons/service_logger/service_logger.py", line 97, in call_kw
      return self._call_kw(model, method, args, kwargs)
    File "/opt/odoo/addons/service_logger/service_logger.py", line 83, in _call_kw
      res = super(LoggerDataSet, self)._call_kw(model, method, args, kw)
    File "/opt/odoo/addons/web/controllers/main.py", line 936, in _call_kw
      return getattr(request.registry.get(model), method)(request.cr, request.uid, *args, **kwargs)
    File "/opt/odoo/openerp/api.py", line 256, in wrapper
      return old_api(self, *args, **kwargs)
    File "/opt/odoo/openerp/api.py", line 387, in old_api
      result = method(recs, *args, **kwargs)
    File "/opt/odoo/openerp/models.py", line 5957, in onchange
      record._onchange_eval(name, field_onchange[name], result)
    File "/opt/odoo/openerp/models.py", line 5857, in _onchange_eval
      params = eval("[%s]" % params, global_vars, field_vars)
    File "/opt/odoo/openerp/tools/safe_eval.py", line 314, in safe_eval
      return eval(c, globals_dict, locals_dict)
    File "", line 1, in &lt;module&gt;
  ValueError: "name 'partner_id' is not defined" while evaluating
  '[partner_id]'
  </code></pre>