Django Models

Posted 2020-08-12

所有对model object的操作都会根据model的定义翻译成SQL。

 

Relationship 怎么定义：

ManyToOne (和OneToMany是一样的):   ForeignKey ， One side access Many: one.many_set (relatedManager)

OneToOne:　　OneToOneField

ManyToMany: ManyToManyField ， 只在一边定义(many.many)，在没有定义的另一边访问时：many.many_set (ManyRelatedManager)

 

OneToOne 用 inner join

ManyTonOne 用left join

OneToMany 用独立的select ... in ()

ManyToMany 用独立的select ... inner join on  ... in()

 

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合理利用cache， 避免每次用到query_set时都要访问数据库

 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 (需要另一条select去数据)

OneToOne (ManyToOne):  select_related( 同一条select 加join即可取出所有数据时使用）

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如何打印出SQL？

from django.db import connection

print connection.queries

 

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bulk insert 怎么做效率最高？

三种方法：

1. bulk_create  (还是使用django model save, 一次Insert， 一个transaction)

2. transaction.atomic()  (一个transaction,多次save)

3. cursor (自己写sql, 绕过django model)

 

第三个方案是最快的应该