title: Database table not deleted after uninstalling module in odoo 9
body: <p>I have a custom module in odoo 9. In the model, I have animals and cages. </p>
  
  <p>In a cage can be more than one animal.
  An animal can be only in one cage.</p>
  
  <p>That is my model:</p>
  
  <pre><code>class Animal(osv.osv):
      _name = 'Animal'
      _columns = {
                  'name': fields.char('Animal Name', size=2048),
                  'cage_id': fields.many2one('cage', required=True, ondelete='cascade', string="Cage"),
      }
  
  class Cage(osv.osv):
      _name = 'Cage'
      _columns = {
                  'name': fields.char('Cage Name', size=100),
                  'animals': fields.one2many('Animal', 'cage_id', string="Animals"),
                  }
  </code></pre>
  
  <p>When I install the module, everything works fine. But if I try to uninstall it, the table "Animal" remains in the database and there is an error in the console:</p>
  
  <pre><code>2016-10-31 16:37:21,091 28082 INFO myserver openerp.addons.base.ir.ir_model: Unable to delete 7565@ir.model.fields
  Traceback (most recent call last):
    File "/etc/odoo/server/openerp/addons/base/ir/ir_model.py", line 1269, in unlink_if_refcount
      self.pool[model].unlink(cr, uid, [res_id], context=context)
    File "/etc/odoo/server/openerp/api.py", line 250, in wrapper
      return old_api(self, *args, **kwargs)
    File "/etc/odoo/server/openerp/addons/base/ir/ir_model.py", line 461, in unlink
      self.browse(cr, user, ids, context=context)._prepare_update()
    File "/etc/odoo/server/openerp/api.py", line 248, in wrapper
      return new_api(self, *args, **kwargs)
    File "/etc/odoo/server/openerp/addons/base/ir/ir_model.py", line 449, in _prepare_update
      raise UserError(msg % (field, model._field_inverses[field][0]))
  UserError: (u"The field 'animal.cage_id' cannot be removed because the field 'cage.passwords' depends on it.", None)
  </code></pre>
  
  <p>I have reviewed the official documentation, but I am not able to locate the error.</p>
  
  <p>What is wrong in the model?</p>