Django的數據庫查詢優化

在Django的數據庫查詢優化新手問題：Django的數據庫查詢優化

我有一個自定義模式表單編輯一個目標對象，我在構造函數中，我得到一個QuerySet的持有從相關訪客模式，具有多對多字段到目的地（見編輯使用自定義模型形式的原因）

print "loading initial choices" 
    visitor_choices, visitor_initial = [], [] 
    visitor_set = self.instance.visitor_set.all() 
    print visitor_set 
    for obj in Visitor.objects.all(): 
     visitor_choices.append((obj.pk, obj.name)) 
     #if visitor_set.filter(pk=obj.pk # this hits the db every time! 
     if obj in visitor_set: 
      visitor_initial.append(obj.pk) 

    self.fields['visitors'].choices = visitor_choices 
    self.fields['visitors'].initial = visitor_initial 

    print "finished loading initial choices"

的想法是相關visitor_set加載到一個變量，以避免重複的查詢，以檢查是否每個訪問者存在於visitor_set。這是最好的方法嗎？我可以看到一個重複的查詢（第三個SELECT語句）選擇目標ID爲1的所有訪問者，但是這並不存在於我寫的代碼，它從哪裏來的？

loading initial choices 
(0.000) SELECT "testapp_visitor"."id", "testapp_visitor"."name" FROM "testapp_visitor" INNER JOIN "testapp_visitor_destinations" ON ("testapp_visitor"."id" = "testapp_visitor_destinations"."visitor_id") WHERE "testapp_visitor_destinations"."destination_id" = 1 LIMIT 21; args=(1, 
) 
[<Visitor: MIMA>, <Visitor: MIMO>, <Visitor: MIMU>] 
(0.000) SELECT "testapp_visitor"."id", "testapp_visitor"."name" FROM "testapp_visitor"; args=() 
(0.000) SELECT "testapp_visitor"."id", "testapp_visitor"."name" FROM "testapp_visitor" INNER JOIN "testapp_visitor_destinations" ON ("testapp_visitor"."id" = "testapp_visitor_destinations"."visitor_id") WHERE "testapp_visitor_destinations"."destination_id" = 1 ; args=(1,) 
finished loading initial choices

EDIT

Destination的對象我指的是一個ManyToMany場的Visitor物體上的相關側。如果我的表單正在編輯Visitor對象本身，那麼Django會自動處理ManyToMany字段。但要在Destination的模型表單上執行此操作，我需要爲Visitor添加多選字段，並自定義__init__方法以加載它的選項和初始選擇。

問題不過是關於如何處理查詢集，以及神祕的第二個SQL加載多對多值，我也可以從外殼看到：

>>> from testapp.forms import DestinationForm 
>>> from testapp.models import Destination, Visitor 
>>> dest = Destination.objects.get(pk=1) 
(0.001) SELECT "testapp_destination"."id", "testapp_destination"."destination" FROM "testapp_destination" WHERE "testapp_destination"."id" = 
1 ; args=(1,) 
>>> destinationForm = DestinationForm(instance=dest) 
loading initial choices 
(0.000) SELECT "testapp_visitor"."id", "testapp_visitor"."name" FROM "testapp_visitor" INNER JOIN "testapp_visitor_destinations" ON ("testap 
p_visitor"."id" = "testapp_visitor_destinations"."visitor_id") WHERE "testapp_visitor_destinations"."destination_id" = 1 LIMIT 21; args=(1, 
) 
[<Visitor: MIMA>, <Visitor: MIMO>, <Visitor: MIMU>] 
(0.000) SELECT "testapp_visitor"."id", "testapp_visitor"."name" FROM "testapp_visitor"; args=() 
(0.000) SELECT "testapp_visitor"."id", "testapp_visitor"."name" FROM "testapp_visitor" INNER JOIN "testapp_visitor_destinations" ON ("testap 
p_visitor"."id" = "testapp_visitor_destinations"."visitor_id") WHERE "testapp_visitor_destinations"."destination_id" = 1 ; args=(1,) 
finished loading initial choices 
>>>

感謝

來源

2011-11-30 xuloChavez

你能解釋你想達到什麼嗎？對我來說，如果你爲其中'visitor'是'ManyToManyField'的模型生成一個'ModelForm'，你就會默認做django應該做的事情？ –

是的，你是對的，我的表單試圖保存一個'目的地'模型，它位於我的'訪問者'模型的ManytoMany字段的相關端，我將發佈模型來澄清。所以我可以有一個表格來保存'訪問者'，它會自動處理ManyToMany，但是我想要探索這種做法。然而，我的問題是關於我處理查詢集的方式，以及我在日誌中看到的奇怪的附加sql語句 – xuloChavez

要回答什麼你說是你的問題：我想查詢

SELECT "testapp_visitor"."id", "testapp_visitor"."name" FROM "testapp_visitor" INNER JOIN "testapp_visitor_destinations" ON ("testapp_visitor"."id" = "testapp_visitor_destinations"."visitor_id") WHERE "testapp_visitor_destinations"."destination_id" = 1 ; args=(1,)

來自行

if obj in visitor_set:

其中Django的重新執行的visitor_set查詢（參見Django文檔When QuerySets are evaluated）。您可以避免通過轉換visitor_set到set馬上（這樣的Django被迫立即執行查詢），如：

visitor_set = set(self.instance.visitor_set.all())

這也將一個對象是否在這組提高測試的性能（與list或類似的迭代相比）。

來源

2011-12-14 10:02:08

Django的數據庫查詢優化

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