title: Create a model User that inherit res.users
body: <p>I would like to create a model "my_model.user" that inherit "res.users", but I have a problem and I don't understand it !</p>
  
  <pre><code>from openerp import models, fields, api
  
  class User(models.Model):
      _name = 'my_model.user'
      _inherit = 'res.users'
  
      name = fields.Char(string="Name", required=True)
      user_first_name = fields.Char(string="First name", required=True)
      user_flight_hours = fields.Integer(string="Flight hours", default=0)
      #code...
  </code></pre>
  
  <p>And here is my error message :</p>
  
  <pre><code>Integrity Error
  
  The operation cannot be completed, probably due to the following:
  - deletion: you may be trying to delete a record while other records still    reference it
  - creation/update: a mandatory field is not correctly set
  
  [object with reference: Users - res.users]
  </code></pre>
  
  <p>Could you help me ?</p>