Advanced

Raw SQL 

from django.db import connection
with connection.cursor() as cursor:
    cursor.execute("SELECT foo FROM bar WHERE baz = %s", [self.baz])
    row = cursor.fetchone()
    print(row)

Transactions 事务

```
from django.db import transaction

@transaction.atomic
def viewfunc(request):
    do_stuff()

with transaction.atomic():
    do_more_stuff()
```

[ ] Managers

[ ] Custom lookups

Multiple databases 

Query Expressions

官网一些可以计算出结果，并用来过滤修改的方法。

Database Functions 

Math Functions

Abs: 求绝对值

流水总量
Transaction.objects.aggregate(Sum(Abs('amount')))

`F() expressions`

F() 代表了一个从model获取的数据，但是不会取出到python内存里。

例子

reporter = Reporters.objects.get(name="Tintin")
reporter.stories_filed += 1  # 取出数据，放入python，加1
reporter.save()  # 保存到sql

from django.db.models import F
reporter.stories_filed = F('stories_filed') + 1
reporter.save()
reporter.stories_filed  # <CombinedExpression: F(stories_filed) + Value(1)>
# 如果需要获取数据，要使用
reporter.refresh_from_db()

reporter = Reporters.objects.filter(name="Tintin")
reporter.update(stories_filed=F("stories_filed") + 1)

通过F可以避免多线程操作同一个数据字段的问题
注意，每次save时，F都会计算一遍，都会加1.如果不想这么做，请save后refresh_from_db
这个字段可以用在queries, 参见 Filters can reference fields on the model
[ ] 用在annotations
[ ] 用在null来排序

Database Functions 

Math Functions

Abs: 求绝对值

流水总量
Transaction.objects.aggregate(Sum(Abs('amount')))