Odoo - 从many2one中获取单个第一个对象

Question

我有一个名为cmms.history和cmms.equipment的类，我想显示分配给cmms.equipment的cmms.history中的第一个元素，但我有以下错误“ValueError：字典更新序列元素＃0的长度为1; 2是必需的“

class cmms_history(osv.osv):
_name = 'cmms.history'
_description = 'CMMS History'
_columns = {
    'equipment_id': fields.many2one('cmms.equipment','Unit of work ref', required=True),
    'customer': fields.many2one('res.partner', 'Customer',required=True),
    'start_date': fields.date("Start Date"),
    'end_date': fields.date("End Date"),        

}    
cmms_history()

class cmms_equipment(osv.osv):
_name = "cmms.equipment"
_description = "equipment"
_inherit = ['mail.thread','ir.needaction_mixin','cmms.history']  
_columns = {
    'main_location': fields.function(_last_name,type='many2one', string='Location',  readonly = True),

}
  def last_location(self, cr, uid, ids, field_name, arg, context=None): 
    return self.pool.get('cmms.history').browse(cr, uid, ids)   
cmms_equipment()

我想在设备类的历史记录中显示客户。

感谢。