I can't figure out what's wrong with my related Many2one field in the ORM API.

If I do the same with Boolean or Float fields it works perfect, but with the Many2one field I get a key error when I open the form with this fields. Without opening a form no error occurs.

Code that works:

account_type = fields.Selection(
    'Account Type', related='template_id.account_type',
     readonly=True)
external_db = fields.Boolean(
     'External DB', related='template_id.external_db',
      readonly=True) 

Code that doesn't work:

type_prefix = fields.Many2one(
    'Account Type Prefix', related='template_id.type_prefix',
     readonly=True)
currency_base = fields.Many2one(
    'Currency', related='template_id.currency_id',
     readonly=True) 

A bit special is that the the related model is in OSV instead:

'type_prefix': fields.many2one(
    'exchange.account.type', 'Account Number Prefix/Type', required=False, size=2,
    help="Prefix for Number of the Accounts"), 
'currency_id': fields.many2one(
    'res.currency', 'Currency', required=True),