Django Models

全部對model object的操做都會根據model的定義翻譯成SQL。

 

Relationship 怎麼定義：

ManyToOne (和OneToMany是同樣的):   ForeignKey ， One side access Many: one.many_set (relatedManager)

OneToOne:　　OneToOneField

ManyToMany: ManyToManyField ， 只在一邊定義(many.many)，在沒有定義的另外一邊訪問時：many.many_set (ManyRelatedManager)

 

OneToOne 用 inner join

ManyTonOne 用left join

OneToMany 用獨立的select ... in ()

ManyToMany 用獨立的select ... inner join on  ... in()

 

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合理利用cache， 避免每次用到query_set時都要訪問數據庫

 a = models.Customer.objects.all().prefetch_related('products')

// no query happen at all

b = list(a)

// both Customer and products have been retrieved and cached.

a = models.Customer.objects.all()

b = list(a)

// only Customer has been fetched.

 

for i in b:

　　print i.products  // another query to database to fetch current products, not all products of every customer.

 

ManyToMany (OneToMany) : prefetch_related (須要另外一條select去數據)

OneToOne (ManyToOne):  select_related( 同一條select 加join便可取出全部數據時使用）

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如何打印出SQL？

from django.db import connection

print connection.queries

 

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bulk insert 怎麼作效率最高？

三種方法：

1. bulk_create  (仍是使用django model save, 一次Insert， 一個transaction)

2. transaction.atomic()  (一個transaction,屢次save)

3. cursor (本身寫sql, 繞過django model)

 

第三個方案是最快的應該