Question

为了方便起见，我正在使用 raw_sql 查询，以便使数据库最小化。我正在删除多余的记录。通过该查询

#d is from a loop and has values
res=MyModel.objects.raw("DELETE FROM mydb_mymodel WHERE mydb_mymodel.s_type = '%s' and mydb_mymodel.barcode = '%s' and mydb_mymodel.shopcode = '%s' and mydb_mymodel.date = '%s'" ,[d.s_type,d.barcode,d.shopcode,d.date])

它不是删除数据库中的记录，而是

当我执行res.query并从postgres控制台运行它时，它将起作用！

是的，我可以使用

MyModel.objects.filter(s_type=d.s_type,barcode=d.barcode,
shopcode=d.shopcode,date=d.date).delete()

但是我在 raw_sql 中缺少什么？

Answer 1

.raw(..)不会急于执行，就像大多数Django ORM查询是懒惰地执行一样。因此，它返回一个RawQuerySet对象，并在对象中进行查询。例如：

>>> User.objects.raw('BLA BLA BLA', [])
<RawQuerySet: BLA BLA BLA>

诸如BLA BLA BLA之类的查询没有任何意义：数据库将在其上出错，但仍然可以检索到RawQuerySet。

您可以通过例如迭代评估来强制评估，然后我们得到：

>>> list(User.objects.raw('BLA BLA BLA', []))
Traceback (most recent call last):
  File "/djangotest/env/lib/python3.6/site-packages/django/db/backends/utils.py", line 85, in _execute
    return self.cursor.execute(sql, params)
  File "/djangotest/env/lib/python3.6/site-packages/django/db/backends/mysql/base.py", line 71, in execute
    return self.cursor.execute(query, args)
  File "/djangotest/env/lib/python3.6/site-packages/MySQLdb/cursors.py", line 250, in execute
    self.errorhandler(self, exc, value)
  File "/djangotest/env/lib/python3.6/site-packages/MySQLdb/connections.py", line 50, in defaulterrorhandler
    raise errorvalue
  File "/djangotest/env/lib/python3.6/site-packages/MySQLdb/cursors.py", line 247, in execute
    res = self._query(query)
  File "/djangotest/env/lib/python3.6/site-packages/MySQLdb/cursors.py", line 412, in _query
    rowcount = self._do_query(q)
  File "/djangotest/env/lib/python3.6/site-packages/MySQLdb/cursors.py", line 375, in _do_query
    db.query(q)
  File "/djangotest/env/lib/python3.6/site-packages/MySQLdb/connections.py", line 276, in query
    _mysql.connection.query(self, query)
_mysql_exceptions.ProgrammingError: (1064, "You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near 'BLA BLA BLA' at line 1")

因此list(..)会强制求值，现在数据库当然会产生错误。但是，即使这是有效的 DELETE 查询，它仍然会引发错误，因为此类查询不会返回任何记录。

为了进行DELETE调用，Django手册指定您应该use a cursor [Django-doc]：

from django.db import connection

with connection.cursor() as cursor:
    cursor.execute(
        "DELETE FROM mydb_mymodel WHERE s_type = '%s' AND barcode = '%s' AND shopcode = '%s' AND date = '%s'" ,
        [d.s_type,d.barcode,d.shopcode,d.date]
    )

但是我认为指定它可能要简单得多：

MyModel.objects.filter(
    s_type=d.s_type,
    barcode=d.barcode,
    shopcode=d.shopcode,
    date=d.date
).delete()

这将构造一个DELETE查询，并正确地序列化参数。一个.delete()查询很急切，因此犯上述错误的几率要低得多：如果ORM正确实现，那么我们就不必担心。

Django ORM原始删除查询不删除记录

1 个答案: