Django ORM 进行查询操作和性能优化

一, ORM 的基本操作

测试数据

from django.db import models


class Publisher(models.Model):
　　# 出版社
    name = models.CharField(max_length=32)
    addr = models.CharField(max_length=32)
    phone = models.IntegerField

    def __str__(self):
        return self.name



class Author(models.Model):
 　　# 作者
    name = models.CharField(max_length=16)
    author_detail = models.OneToOneField("AuthorDetail")
    books = models.ManyToManyField(to="Book")

    def __str__(self):
        return self.name


class Book(models.Model):
　　# 书籍信息
    title = models.CharField(max_length=6)
    price = models.DecimalField(max_digits=5, decimal_places=2)
    publish_day = models.DateField(auto_now_add=True)
    publisher = models.ForeignKey(to="Publisher", to_field="id")

    def __str__(self):
        return self.title


class AuthorDetail(models.Model):
　　# 作者详情
    city = models.CharField(max_length=32)
    email = models.EmailField()

    def __str__(self):
        return self.city

增
 
models.Tb1.objects.create(c1='xx', c2='oo')  增加一条数据，可以接受字典类型数据 **kwargs
 
obj = models.Tb1(c1='xx', c2='oo')
obj.save()
 
 查
 
models.Tb1.objects.get(id=123)         # 获取单条数据，不存在则报错（不建议）
models.Tb1.objects.all()               # 获取全部
models.Tb1.objects.filter(name='seven') # 获取指定条件的数据
models.Tb1.objects.exclude(name='seven') # 获取指定条件的数据
 
 删
 
models.Tb1.objects.filter(name='seven').delete() # 删除指定条件的数据
 
 改
models.Tb1.objects.filter(name='seven').update(gender='0')  # 将指定条件的数据更新，均支持 **kwargs
obj = models.Tb1.objects.get(id=1)
obj.c1 = '111'
obj.save()    

二, ForeignKey 的使用原因

 约束
 节省硬盘
 但是多表查询会降低速度，大型程序反而不使用外键，而是用单表(约束的时候，通过代码判断)

extra

extra(self, select=None, where=None, params=None, tables=None, order_by=None, select_params=None)
   Entry.objects.extra(select={'new_id': "select col from sometable where othercol > %s"}, select_params=(1,))
   Entry.objects.extra(where=['headline=%s'], params=['Lennon'])
   Entry.objects.extra(where=["foo='a' OR bar = 'a'", "baz = 'a'"])
   Entry.objects.extra(select={'new_id': "select id from tb where id > %s"}, select_params=(1,), order_by=['-nid'])

F与Q查询

from django.db.models import F
models.Tb1.objects.update(num=F('num')+1)

    
Book.objects.all().update(price=F("price")+30)　

from django.db.models import Q
con = Q()
 
q1 = Q()
q1.connector = 'OR'
q1.children.append(('id', 1))
q1.children.append(('id', 10))
q1.children.append(('id', 9))
 
q2 = Q()
q2.connector = 'OR'
q2.children.append(('c1', 1))
q2.children.append(('c1', 10))
q2.children.append(('c1', 9))
 
con.add(q1, 'AND')
con.add(q2, 'AND')
 
models.Tb1.objects.filter(con)
 
from django.db import connection
cursor = connection.cursor()
cursor.execute("""SELECT * from tb where name = %s""", ['Lennon'])
row = cursor.fetchone()

select_related(self, *fields)

性能相关：表之间进行join连表操作，一次性获取关联的数据。
    model.tb.objects.all().select_related()
    model.tb.objects.all().select_related('外键字段')
    model.tb.objects.all().select_related('外键字段__外键字段')

prefetch_related(self, *lookups)

性能相关：多表连表操作时速度会慢，使用其执行多次SQL查询  在内存中做关联，而不会再做连表查询
           # 第一次 获取所有用户表
           # 第二次 获取用户类型表where id in (用户表中的查到的所有用户ID)
           models.UserInfo.objects.prefetch_related('外键字段')

annotate(self, *args, **kwargs)(聚合函数)

 from django.db.models import Count, Avg, Max, Min, Sum
 
    v = models.UserInfo.objects.values('u_id').annotate(uid=Count('u_id'))
    # SELECT u_id, COUNT(ui) AS `uid` FROM UserInfo GROUP BY u_id
 
    v = models.UserInfo.objects.values('u_id').annotate(uid=Count('u_id')).filter(uid__gt=1)
    # SELECT u_id, COUNT(ui_id) AS `uid` FROM UserInfo GROUP BY u_id having count(u_id) > 1
 
    v = models.UserInfo.objects.values('u_id').annotate(uid=Count('u_id',distinct=True)).filter(uid__gt=1)
   # SELECT u_id, COUNT( DISTINCT ui_id) AS `uid` FROM UserInfo GROUP BY u_id having count(u_id) > 1

extra(self, select=None, where=None, params=None, tables=None, order_by=None, select_params=None)

 Entry.objects.extra(select={'new_id': "select col from sometable where othercol > %s"}, select_params=(1,))
      Entry.objects.extra(where=['headline=%s'], params=['Lennon'])
      Entry.objects.extra(where=["foo='a' OR bar = 'a'", "baz = 'a'"])
      Entry.objects.extra(select={'new_