django模型方法extra

## select提供简单数据
# SELECT age, (age > 18) as is_adult FROM myapp_person;
Person.objects.all().extra(select={'is_adult': "age > 18"})  # 加在select后面

## where提供查询条件
# SELECT * FROM myapp_person WHERE first||last ILIKE 'jeffrey%';
Person.objects.all().extra(where=["first||last ILIKE 'jeffrey%'"])  # 加一个where条件

## table连接其它表
# SELECT * FROM myapp_book, myapp_person WHERE last = author_last
Book.objects.all().extra(table=['myapp_person'], where=['last = author_last']) # 加from后面

## params添参数
# !! 错误的方式 !!
first_name = 'Joe'  # 如果first_name中有SQL特定字符就会出现漏洞
Person.objects.all().extra(where=["first = '%s'" % first_name])
# 正确方式
Person.objects.all().extra(where=["first = '%s'"], params=[first_name])

 extra源码

def extra(self, select=None, where=None, params=None, tables=None,
              order_by=None, select_params=None):
        """
        Adds extra SQL fragments to the query.
        """
        assert self.query.can_filter(), \
                "Cannot change a query once a slice has been taken"
        clone = self._clone()
        clone.query.add_extra(select, select_params, where, params, tables, order_by)
        return clone

 原文：https://www.cnblogs.com/songbird/p/7562923.html

#相关子查询，而且只能select一个值(a.id),否则报错:当没有用 EXISTS 引入子查询时，在选择列表中只能指定一个表达式
InnerMail.objects.extra(select={'status':'select a.id from letter_imail_status a where a.id=letter_innermail.id'})