如何在 sqlalchemy 中使用子查询添加行? [英] How to Add rows using subqueries in sqlalchemy?

问题描述

我正在使用带有 SQLAlchemy 的 Postgresql，但似乎 sqlalchemy 在使用子查询时无法添加行.

I'm using Postgresql with SQLAlchemy but it seems sqlalchemy is having trouble adding rows when using subqueries.

在我的示例中，我想为表中的特定标签更新计数器.

In my example, I want to update a counter for a specific tag in a table.

在 SqlAlchemy 中，测试运行类如下所示:

In SqlAlchemy a test run class would look like the following:

class TestRun( base ):
    __tablename__   = 'test_runs'

    id              = sqlalchemy.Column( 'id', sqlalchemy.Integer, sqlalchemy.Sequence('user_id_seq'), primary_key=True )
    tag             = sqlalchemy.Column( 'tag', sqlalchemy.String )
    counter         = sqlalchemy.Column( 'counter', sqlalchemy.Integer )

插入代码应如下所示:

tag = 'sampletag'
counterquery = session.query(sqlalchemy.func.coalesce(sqlalchemy.func.max(TestRun.counter),0) + 1).
                            filter(TestRun.tag == tag).
                            subquery()

testrun = TestRun()
testrun.tag = tag
testrun.counter = counterquery

session.add( testrun )
session.commit()

这个问题是，它在运行这段代码时给出了一个非常有趣的错误，它正在尝试运行以下 SQL 查询:

The problem with this, is it gives a very interesting error when running this code, it's trying to run the following SQL Query:

'INSERT INTO test_runs (id, tag, counter) 
    VALUES (%(id)s, 
            %(tag)s, 
            SELECT coalesce(max(test_runs.counter), %(param_1)s) + %(coalesce_1)s AS anon_1 
               FROM test_runs 
               WHERE test_runs.tag = %(tag_1)s)' 
 {'coalesce_1': 1, 'param_1': 0, 'tag_1': 'mytag', 'tag': 'mytag', 'id': 267L}

这看起来很合理，只是在 SELECT 调用周围缺少括号.当我手动运行 SQL 查询时，它给了我与 sqlalchemy 给我的完全相同的错误，直到我手动输入括号来修复所有问题.似乎是一个不太可能的错误，sqlalchemy 在需要时会忘记放括号，所以我的问题是我是否缺少在使用 sqlalchemy 添加行时正确使用子查询的函数?

Which looks reasonable, except it's missing parenthesis around the SELECT call. When I run the SQL query manually it gives me the same exact error that sqlalchemy gives me until I type in the parenthesis manually which fixes everything up. Seems like an unlikely bug that sqlalchemy would forget to put parenthesis when it needs to, so my question is am I missing a function to use subqueries correctly when adding rows using sqlalchemy?

推荐答案

而不是使用 subquery() 调用 as_scalar() 方法:

Instead of using subquery() call as_scalar() method:

返回此 Query 表示的完整 SELECT 语句，转换后到一个标量子查询.

Return the full SELECT statement represented by this Query, converted to a scalar subquery.

示例:

具有分类父子关系的模型:

Models with classing parent-child relationship:

class Parent(Base):
    __tablename__ = 'parents'
    id = Column(Integer, primary_key=True)
    counter = Column(Integer, nullable=False, default=0)

class Child(Base):
    __tablename__ = 'children'
    id = Column(Integer, primary_key=True)
    parent_id = Column(ForeignKey(Parent.id), nullable=False)
    parent = relationship(Parent)

更新counter字段的代码:

parent.counter = session.query(func.count(Child.id))
                    .filter_by(parent=parent).as_scalar()

产生的 SQL(从日志中复制):

Produced SQL (copied from the log):

UPDATE parents SET counter=(SELECT count(children.id) AS count_1 
FROM children
WHERE ? = children.parent_id) WHERE parents.id = ?

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