Django ORM操做补充

操做补充

only 只取某些去除其余 

defer 去除某些取其余

# 需求: 只取某n列
queryset=[ {},{}]
models.User.objects.all().values( 'id','name')

queryset=[ (),()]
models.User.objects.all().values_list( 'id','name')

queryset=[ obj,obj]
result = models.User.objects.all().only('id','name','age')
# result = models.User.objects.all().defer('id','name','age')
for item in reuslt:
print(item.id,item.name,item.age)

selected_related 主动连表查询 通常这个就够用了

prefetch_related 进阶版连表查询 最优方案

# 需求: 打印全部用户姓名以及部门名称
    class depart:
        title = ....
    class User:
        name = ...
        dp = FK(depart)
    # select * from user
    # result = models.User.objects.all()
    # for item in result:    # 性能低，会跨表 至关于你查10我的就跨表查了10次+原本1次
    # 可是若是连表太多。那性能也会变差，由于表太多了
    #     print(item.name,item.dp.title)


    # select * from user left join depart on user.dp_id = depart.id
    # result = models.User.objects.all().selected_related('dp') # 主动建立了关联关系。一次查表就能够拿出来
    # for item in result: # 就不会在发请求查询了，性能更好了
    #     print(item.name,item.dp.title )


    # select * from user
    # 经过python 代码获取 dp_id= [1,2]
    # select * from depart where id in dp_id
    result = models.User.objects.all().prefetch_related('dp')
    for item in result: # 至关于查了两次。可是不会由于连表太多影响性能了
        print(item.name,item.dp.title )

 经过ORM写偏原生SQL

# - extra
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'])


"""----------------------------------- """


# - raw
    # 执行原生SQL
models.UserInfo.objects.raw('select * from userinfo')
    # 若是SQL是其余表时，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 其余表')
    # 为原生SQL设置参数
models.UserInfo.objects.raw('select id as nid from userinfo where nid>%s', params=[12,])
    # 指定捆绑条件后跨表查询
name_map = {'first': 'first_name', 'last': 'last_name', 'bd': 'birth_date', 'pk': 'id'}
Person.objects.raw('SELECT * FROM some_other_table', translations=name_map)


"""----------------------------------- """


# - 彻底执行原生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(..)
    # PS: 选择数据库 using('default') 默认是  default
queryset = models.Course.objects.using('default').all()

获取model表名字

model_name = obj.queryset.model._meta.model_name

 

获取app名字

app_label = obj.queryset.model._meta.app_label

 

 根据字符串取出来每一个model字段对象

title_obj = field_obj._meta.get_field("title")
title_max_length = title.max_length      # 32     # 能够拿出来字段的属性对应的值
title.rel.to.objects.all()     # 根据字段对象可取出来映射关联表的全部内容

values 的本质

循环每一个字段，生成字典 字段为键，查询的为值，(字典一个键只能有一个值) 所以会有重复数据
用 .distinctu() 去重