select_for_update
- 返回queryset，并将需要更新的行锁定，类似于
SELECT ... FOR UPDATE的操作。
# 所有匹配的entries都会被锁定直到此次事务结束。
with transaction.atomic():
entries = Entry.objects.select_for_update().filter(author=request.user)
select_related
from django.db import models
class Province(models.Model):
name = models.CharField(max_length=10)
def __unicode__(self):
return self.name
class City(models.Model):
name = models.CharField(max_length=5)
province = models.ForeignKey(Province)
def __unicode__(self):
return self.name
-
传统做法
>>> citys = City.objects.all() >>> for c in citys: ... print c.province ...这样会导致线性的SQL查询，如果对象数量n太多，每个对象中有k个外键字段的话，就会导致n*k+1次SQL查询。在本例中，因为有3个city对象就导致了4次SQL查询：
SELECT `QSOptimize_city`.`id`, `QSOptimize_city`.`name`, `QSOptimize_city`.`province_id` FROM `QSOptimize_city`; SELECT `QSOptimize_province`.`id`, `QSOptimize_province`.`name` FROM `QSOptimize_province` WHERE `QSOptimize_province`.`id` = 1 ; SELECT `QSOptimize_province`.`id`, `QSOptimize_province`.`name` FROM `QSOptimize_province` WHERE `QSOptimize_province`.`id` = 2 ; SELECT `QSOptimize_province`.`id`, `QSOptimize_province`.`name` FROM `QSOptimize_province` WHERE `QSOptimize_province`.`id` = 1 ; -
select_related做法
>>> citys = City.objects.select_related('province').all() >>> for c in citys: ... print c.province ...SELECT `QSOptimize_city`.`id`, `QSOptimize_city`.`name`, `QSOptimize_city`.`province_id`, `QSOptimize_province`.`id`, `QSOptimize_province`.`name` FROM`QSOptimize_city` INNER JOIN `QSOptimize_province` ON (`QSOptimize_city`.`province_id` = `QSOptimize_province`.`id`) ; 注意使用select_related一定要指定参数或者指定
depth，否则默认depth=0，将会遍历该model的所有外键，知道遍历不到。
get_or_created
obj, created = Person.objects.get_or_create(
first_name='John',
last_name='Lennon',
defaults={'birthday': date(1940, 10, 9)},
)
if created:
# means you have created a new person
else:
# person just refers to the existing one
Any keyword arguments passed to get_or_create() — except an optional one called defaults — will be used in a get() call. If an object is found, get_or_create() returns a tuple of that object and False. If multiple objects are found, get_or_create raises MultipleObjectsReturned. If an object is not found, get_or_create() will instantiate and save a new object, returning a tuple of the new object and True.
bulk_create
在Django中需要向数据库中插入多条数据（list）。使用如下方法，每次save()的时候都会访问一次数据库。导致性能问题:
for i in resultlist:
p = Account(name=i)
p.save()
在django1.4以后加入了新的特性。使用django.db.models.query.QuerySet.bulk_create()批量创建对象，减少SQL查询次数。改进如下：
querysetlist=[]
for i in resultlist:
querysetlist.append(Account(name=i))
Account.objects.bulk_create(querysetlist)
values_list('', flat=True)
- 该查询会获得一个
django.db.models.query.ValuesListQuerySet类型的变量。如果想转化成LIST类型的，可以用下面粗暴的方法：picture_ids = list(picture_foreign_pictures.values_list('picture_id', flat=True))
