如何在 SQLAlchemy 中增加计数器 [英] How to increase a counter in SQLAlchemy

问题描述

假设我有一个 tags 表，它有一个 count 字段，表示有多少 items 用给定的标签进行了标记.

Suppose I have table tags which has a field count that indicates how many items have been tagged with the given tag.

在添加带有现有标记的新项目后，如何在 SQLAlchemy 中增加此计数器?

How do I increase this counter in SQLAlchemy after I add a new item with an existing tag?

使用纯 SQL 我会执行以下操作:

With plain SQL I would do the following:

INSERT INTO `items` VALUES (...)
UPDATE `tags` SET count=count+1 WHERE tag_id=5

但是我如何在 SQLAlchemy 中表达 count=count+1 ?

But how do I express count=count+1 in SQLAlchemy?

推荐答案

如果你有类似的问题:

mytable = Table('mytable', db.metadata,
    Column('id', db.Integer, primary_key=True),
    Column('counter', db.Integer)
)

您可以像这样增加字段:

You can increment fields like this:

m = mytable.query.first()
m.counter = mytable.c.counter + 1

或者，如果你有一些映射的模型，你也可以写成:

Or, if you have some mapped Models, you can write alternatively:

m = Model.query.first()
m.counter = Model.counter + 1

两个版本都会返回你要求的sql语句.但是，如果您不包含该列而只编写 m.counter += 1，那么新值将在 Python 中计算(并且可能会发生竞争条件).因此，请始终在此类计数器查询中包含上面两个示例中所示的列.

Both versions will return the sql statement you have asked for. But if you don't include the column and just write m.counter += 1, then the new value would be calculated in Python (and race conditions are likely to happen). So always include a column as shown in the two examples above in such counter queries.

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