Django過濾外鍵屬性

我想根據外鍵的特定字段的值過濾Django中的表。Django過濾外鍵屬性

我的車型有：

class Retailer(SCOPEModel): 
     """ A business or person that exchanges goods for vouchers with recipients 
     """ 
     office = models.ForeignKey(Office, related_name='retailers') 
     uuid = UUIDField(auto=True, version=4, null=True, help_text=_('unique id')) 
     name = models.CharField(_('Name'), max_length=50, validators=[validate_sluggable], 
                 help_text=_('Name of the retail shop'), blank=False) 
     location = models.ForeignKey(Location, verbose_name=_('location'), blank=True, null=True, 
                   help_text=_('Location of the retail shop'), related_name='retailers') 


class PointOfSaleTerminalAssignment(SCOPEModel): 
     """Point Of Sale (POS) is the location where a transaction occurs for 
     exchange of goods and services 
     and a POS terminal is the hardware used to perform the transactions. 
     These terminals are registered in the system. 
     POS' are managed at office level 
     """ 

     office = models.ForeignKey(Office, related_name='pos_terminals') 
     terminal_type = models.ForeignKey(
       TerminalType, 
       verbose_name=_('Terminal'), 
       help_text=_("Device | Make (model)"), 
     ) 
     wfp_number = models.CharField(
       _('WFP Number'), 
       max_length=50, 
       unique=True, 
       validators=[validate_sluggable], 
       help_text=_("WFP unique generated number e.g. Inventory number") 
     ) 
     serial_number = models.CharField(
       _('Serial Number'), 
       max_length=50, 
       unique=True, 
       help_text=_('Hardware device serial number') 
     ) 
     slug = models.SlugField(
       editable=False, 
       unique=True, 
       help_text=_('Unique ID generated from the WFP number') 
     ) 
     assigned_retailer = models.ForeignKey(
       Retailer, 
       related_name='pos_terminals', 
       null=True, 
       blank=True, 
       help_text=_('Retailer this POS terminal is assigned to') 
     )

我希望得到零售商的詳細信息和分配給他們的POS機的序列號目前我執行兩個查詢：

from maidea.apps.office.models import Office 
from maidea.apps.redemption.models import Retailer, PointOfSaleTerminalAssignment 

office = Office.objects.get(slug='so-co') 
pos = PointOfSaleTerminalAssignment.objects.filter(office=office) 

for p in pos: 
    retailer = p.assigned_retailer 
    print retailer.name

，但我得到這個錯誤，請我究竟做錯了什麼？

來源

2016-09-27 Edward Okech

首先，你的代碼不能產生這樣的錯誤，因爲你要打印'retailer.name'，並在錯誤'NoneType'對象有沒有屬性''location'' –

你assigned_retailer可以爲空和空。因此首先檢查是否有任務。

from maidea.apps.office.models import Office 
from maidea.apps.redemption.models import Retailer, PointOfSaleTerminalAssignment 

pos = PointOfSaleTerminalAssignment.objects.filter(office__slug='so-co') 

for p in pos: 
    if p.assigned_retailer: 
     print p.assigned_retailer.name

來源

2016-09-27 09:28:11 sebb

如果合併兩個查詢到一個，爲什麼你不使用'select_related（）'在一個查詢中選擇'assigned_retailer'，因爲如果你不這樣做，你將會在每個'p.assigned_retailer.name'命中數據庫。 –

真的，豎起大拇指 – sebb

顯然，並非所有PointOfSaleTerminalAssignment實例有一個assigned_retailer，作爲FK領域可以採取NULL值。

但是，您可以安全航行每個retailer的屬性僅如果零售商不None通過與if測試：

for p in pos: 
    retailer = p.assigned_retailer 
    if retailer: 
     print retailer.name

來源

2016-09-27 09:26:42

Django過濾外鍵屬性

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