'Return the default value defined for model, field_name, users, company and condition.
Return ``None`` if no such default exists.'

@api.model
def get_default(self, model, field_name, for_all_users=True, company_id=False, condition=False):

    search_criteria = [('key', '=', 'default'), ('key2', '=', (condition and condition[:200])), ('model', '=', model), ('name', '=', field_name), ('user_id', '=', (False if for_all_users else self._uid)), ('company_id', '=', company_id)]
    defaults = self.search(search_criteria)
    return (pickle.loads(defaults.value.encode('utf-8')) if defaults else None)
