Django ORM相关

1. ORM 外键关联查询和多对多关系正反向查询

Class Classes():
    name = CF

class Student():
    name = CF
    class = FK(to="Classes")

class Teacher():
    name = CF
    classes = M2M(to="Classes",related_name="teacher")

简单表结构

    # Fk 正向查找
    obj = models.Student.objects.first()
    print(obj.class_id)

    # FK 反向查找
    obj = models.Classes.objects.first()
    print(obj.student_set.all())

    # M2M 正向查找
    obj = models.Teacher.objects.get(pk=1)
    print(obj.classes.all())

    # M2M 反向查找
    obj = models.Classe.objects.get(pk=1)
    print(obj.teacher.all())

简单实例

2. F查询和Q查询

　　F查询可以对当前查询出来的对象取值,就是像自赋值 a = a+20,更新字段方便

from django.db.models import F
from app01.models import Book

Book.objects.update(price=F("price")+20)  # 对于book表中每本书的价格都在原价格的基础上增加20元

F查询

　　Q查询可以对当前查询的表进行多次筛选,可以组合使用&, | ,比如查询2005年初生且姓是W开头的, 2005年出生或姓名是W

from django.db.models import Q

obj = student.objects.filter(Q(Bd_year=2005) & Q(name_startswith="W"))

obj = student.objects.filter(Q(Bd_year=2005) | Q(name_startswith="W"))

Q查询

3. ORM事务操作

    from app01 import models

    try:
        from django.db import transaction
        with transaction.atomic():
                (进行事务操作)
    except Exception as e:
        pass    

事务简单实例

4. 连表查询提高性能

#select_related, 连表操作, 相当于主动做join

#prefetch_related, 多次单表操作, 先查询想要的数据, 然后构造条件

    如: id[1,2,3],再次查询其他表根据id做条件

连表查询

5. 通过ORM写原生SQL

# - extra  结果集修改器，是吗一种提供额外查询参数的机制
 books= Book.objects.filter(publish='清华出版社').extra(where=['price>50'])

# - raw  执行原始sql并返回模型实例
books = Book.objects.raw('select * from book where publish="清华出版社"')

- 原生SQL
from django.db import connection
cursor=connection.cursor()                                                    # 建立游标                

cursor.execute("insert into book(name) values('小王子')")                        #插入操作

cursor.execute('update book set name='梦里花落知多少' where name='撒哈拉的故事'') #更新操作

cursor.execute('delete from book where name='小王子'')                           #删除操作

cursor.execute('select * from book')                                          #查询操作
raw = cursor.fetchone()                                                       #读取一条
raw_list = cursor.fetchall()                                           #读取所有，返回list
cursor.close()  #关闭游标

原生SQL

6. 自定义管理器

from django.db import models
from django.urls import reverse
from django.utils import timezone
from django.contrib.auth.models import User

class PublishedManager(models.Manager):
    def get_queryset(self):
        return super(PublishedManager, self).get_queryset().filter(status='published')

class Post(models.Model):
    STATUS_CHOICES = (('draft', 'Draft'), ('published', 'Published'))
    title = models.CharField(max_length=250)
    slug = models.SlugField(max_length=250, unique_for_date='publish')
    author = models.ForeignKey(User, on_delete=models.CASCADE, related_name='blog_posts')
    body = models.TextField()
    publish = models.DateTimeField(default=timezone.now)
    created = models.DateTimeField(auto_now_add=True)
    updated = models.DateTimeField(auto_now=True)
    status = models.CharField(max_length=10, choices=STATUS_CHOICES, default='draft')

    objects = models.Manager()  # 默认的管理器
    published = PublishedManager()  # 自定义管理器

    def get_absolute_url(self):
        return reverse('blog:post_detail', args=[self.publish.year, self.publish.month, self.publish.day, self.slug])

    class Meta:
        ordering = ('-publish',)

    def __str__(self):
        return self.title

自定义管理器