Django ORM

ORM性能

1. 查指定字段,使用values()

2. select_related 支持 一对一 多对一 # 连表

3. prefetch_related 支持 多对一 多对多 # 子查询

4. only 只取某些字段 defer 排除某些字段

多个数据库

配置

DATABASES = {
    'default': {
        'ENGINE': 'django.db.backends.sqlite3',
        'NAME': os.path.join(BASE_DIR, 'db.sqlite3'),
    },
    'db2': {
        'ENGINE': 'django.db.backends.sqlite3',
        'NAME': os.path.join(BASE_DIR, 'db2.sqlite3'),
    },

}

 

执行迁移命令

python36 manage.py migrate --database db2

 

读写分离

手动

models.Student.objects.all().using('db2')
obj = models.Student.objects.filter(pk=1).using('db2').update(name='duige')


obj = all_students.first()
obj.name ='duigelaji'
obj.save(using='db2')

自动的方法
class Router():

    def db_for_read(self, model, **kwargs):
        print(model)
        print(kwargs)
        return 'default'

    def db_for_write(self, model, **kwargs):
        print(model)
        print(kwargs)
        return 'db2'
        
settings的配置
DATABASE_ROUTERS = ['router.Router']

一主多从

import random


class Router():

    def db_for_read(self, model, **kwargs):
        return random.choice(['db1', 'db2', 'db3'])

    def db_for_write(self, model, **kwargs):
        print(model)
        print(kwargs)
        return 'default'

分库分表

class Router():

    def db_for_read(self, model, **kwargs):
        app_name = model._meta.app_label

        if app_name == 'app01':
            return 'db1'

        elif app_name == 'app02':
            return 'db2'

    def db_for_write(self, model, **kwargs):
        app_name = model._meta.app_label

        if app_name == 'app01':
            return 'db1'

        elif app_name == 'app02':
            return 'db2'

 

ORM之原生SQL

extra

# extra
# 在QuerySet的基础上继续执行子语句
# extra(self, select=None, where=None, params=None, tables=None, order_by=None, select_params=None)
 
# select和select_params是一组，where和params是一组，tables用来设置from哪个表
# 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'])
 

例子

models.UserInfo.objects.extra(
                    select={'newid':'select count(1) from app01_usertype where id>%s'},
                    select_params=[1,],
                    where = ['age>%s'],
                    params=[18,],
                    order_by=['-age'],
                    tables=['app01_usertype']
                )
                """
                select
                    app01_userinfo.id,
                    (select count(1) from app01_usertype where id>1) as newid
                from app01_userinfo,app01_usertype
                where
                    app01_userinfo.age > 18
                order by
                    app01_userinfo.age desc
                """
 
 
# 执行原生SQL
# 更高灵活度的方式执行原生SQL语句
# from django.db import connection, connections
# cursor = connection.cursor()  # cursor = connections['default'].cursor()
# cursor.execute("""SELECT * from auth_user where id = %s""", [1])
# row = cursor.fetchone()

 

QuerySet方法大全

##################################################################
# PUBLIC METHODS THAT ALTER ATTRIBUTES AND RETURN A NEW QUERYSET #
##################################################################
 
def all(self)
    # 获取所有的数据对象
 
def filter(self, *args, **kwargs)
    # 条件查询
    # 条件可以是：参数，字典，Q
 
def exclude(self, *args, **kwargs)
    # 条件查询
    # 条件可以是：参数，字典，Q
 
def 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。
 
    总结：
    1. select_related主要针一对一和多对一关系进行优化。
    2. select_related使用SQL的JOIN语句进行优化，通过减少SQL查询的次数来进行优化、提高性能。
 
def 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Python代码中实现连表操作。
 
    总结：
    1. 对于多对多字段（ManyToManyField）和一对多字段，可以使用prefetch_related()来进行优化。
    2. prefetch_related()的优化方式是分别查询每个表，然后用Python处理他们之间的关系。
 
def annotate(self, *args, **kwargs)
    # 用于实现聚合group by查询
 
    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
 
def distinct(self, *field_names)
    # 用于distinct去重
    models.UserInfo.objects.values('nid').distinct()
    # select distinct nid from userinfo
 
    注：只有在PostgreSQL中才能使用distinct进行去重
 
def order_by(self, *field_names)
    # 用于排序
    models.UserInfo.objects.all().order_by('-id','age')
 
def extra(self, select=None, where=None, params=None, tables=None, order_by=None, select_params=None)
    # 构造额外的查询条件或者映射，如：子查询
 
    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'])
 
 def reverse(self):
    # 倒序
    models.UserInfo.objects.all().order_by('-nid').reverse()
    # 注：如果存在order_by，reverse则是倒序，如果多个排序则一一倒序
 
 
 def defer(self, *fields):
    models.UserInfo.objects.defer('username','id')
    或
    models.UserInfo.objects.filter(...).defer('username','id')
    #映射中排除某列数据
 
 def only(self, *fields):
    #仅取某个表中的数据
     models.UserInfo.objects.only('username','id')
     或
     models.UserInfo.objects.filter(...).only('username','id')
 
 def using(self, alias):
     指定使用的数据库，参数为别名（setting中的设置）
 
 
##################################################
# PUBLIC METHODS THAT RETURN A QUERYSET SUBCLASS #
##################################################
 
def raw(self, raw_query, params=None, translations=None, using=None):
    # 执行原生SQL
    models.UserInfo.objects.raw('select * from userinfo')
 
    # 如果SQL是其他表时，必须将名字设置为当前UserInfo对象的主键列名
    models.UserInfo.objects.raw('select id as nid from 其他表')
 
    # 为原生SQL设置参数
    models.UserInfo.objects.raw('select id as nid from userinfo where nid>%s', params=[12,])
 
    # 将获取的到列名转换为指定列名
    name_map = {'first': 'first_name', 'last': 'last_name', 'bd': 'birth_date', 'pk': 'id'}
    Person.objects.raw('SELECT * FROM some_other_table', translations=name_map)
 
    # 指定数据库
    models.UserInfo.objects.raw('select * from userinfo', using="default")
 
    ################### 原生SQL ###################
    from django.db import connection, connections
    cursor = connection.cursor()  # cursor = connections['default'].cursor()
    cursor.execute("""SELECT * from auth_user where id = %s""", [1])
    row = cursor.fetchone() # fetchall()/fetchmany(..)
 
 
def values(self, *fields):
    # 获取每行数据为字典格式
 
def values_list(self, *fields, **kwargs):
    # 获取每行数据为元祖
 
def dates(self, field_name, kind, order='ASC'):
    # 根据时间进行某一部分进行去重查找并截取指定内容
    # kind只能是："year"（年）, "month"（年-月）, "day"（年-月-日）
    # order只能是："ASC"  "DESC"
    # 并获取转换后的时间
        - year : 年-01-01
        - month: 年-月-01
        - day  : 年-月-日
 
    models.DatePlus.objects.dates('ctime','day','DESC')
 
def datetimes(self, field_name, kind, order='ASC', tzinfo=None):
    # 根据时间进行某一部分进行去重查找并截取指定内容，将时间转换为指定时区时间
    # kind只能是 "year", "month", "day", "hour", "minute", "second"
    # order只能是："ASC"  "DESC"
    # tzinfo时区对象
    models.DDD.objects.datetimes('ctime','hour',tzinfo=pytz.UTC)
    models.DDD.objects.datetimes('ctime','hour',tzinfo=pytz.timezone('Asia/Shanghai'))
 
    """
    pip3 install pytz
    import pytz
    pytz.all_timezones
    pytz.timezone(‘Asia/Shanghai’)
    """
 
def none(self):
    # 空QuerySet对象
 
 
####################################
# METHODS THAT DO DATABASE QUERIES #
####################################
 
def aggregate(self, *args, **kwargs):
   # 聚合函数，获取字典类型聚合结果
   from django.db.models import Count, Avg, Max, Min, Sum
   result = models.UserInfo.objects.aggregate(k=Count('u_id', distinct=True), n=Count('nid'))
   ===> {'k': 3, 'n': 4}
 
def count(self):
   # 获取个数
 
def get(self, *args, **kwargs):
   # 获取单个对象
 
def create(self, **kwargs):
   # 创建对象
 
def bulk_create(self, objs, batch_size=None):
    # 批量插入
    # batch_size表示一次插入的个数
    objs = [
        models.DDD(name='r11'),
        models.DDD(name='r22')
    ]
    models.DDD.objects.bulk_create(objs, 10)
 
def get_or_create(self, defaults=None, **kwargs):
    # 如果存在，则获取，否则，创建
    # defaults 指定创建时，其他字段的值
    obj, created = models.UserInfo.objects.get_or_create(username='root1', defaults={'email': '1111111','u_id': 2, 't_id': 2})
 
def update_or_create(self, defaults=None, **kwargs):
    # 如果存在，则更新，否则，创建
    # defaults 指定创建时或更新时的其他字段
    obj, created = models.UserInfo.objects.update_or_create(username='root1', defaults={'email': '1111111','u_id': 2, 't_id': 1})
 
def first(self):
   # 获取第一个
 
def last(self):
   # 获取最后一个
 
def in_bulk(self, id_list=None):
   # 根据主键ID进行查找
   id_list = [11,21,31]
   models.DDD.objects.in_bulk(id_list)
 
def delete(self):
   # 删除
 
def update(self, **kwargs):
    # 更新
 
def exists(self):
   # 是否有结果

View Code

 

在Python脚本中调用Django环境

import os
 
if __name__ == '__main__':
    os.environ.setdefault("DJANGO_SETTINGS_MODULE", "BMS.settings")
    import django
    django.setup()
 
    from app01 import models
 
    books = models.Book.objects.all()
    print(books)