如何在Flask-SQLAlchemy中使用count() [英] How to use count() in Flask-sqlalchemy

问题描述

我正在协助一个使用flask-sqlalchemy的项目.我想要一个数据库查询来计算表中的记录数.我可以直接使用table.query.filter_by(condition).count()还是有什么需要我需要添加.请帮助我.我是初学者.谢谢！！

I am assisting a project which uses flask-sqlalchemy.I want a db query to count the number of records in the table.Can I use table.query.filter_by(condition).count() directly or is there anything that I need to add.Please assist me.I am a begineer.Thanks in advance!

推荐答案

给出的答案都没有专门针对 flask-sqlalchemy ，您将在其中使用给出的示例:

None of the given answers address flask-sqlalchemy specifically, where you would use exactly the example you gave:

Table.query.filter_by(condition).count()

您可以在没有过滤器的情况下执行 .count():

You can perform .count() without filters:

Table.query.count()

您还可以使用M2M关系进行计数:

You can also count using M2M relationships:

ParentTable.children.count()

您可以直接在jinja模板中使用其中任何一个，例如:

And you can use any of these directly in your jinja templates like:

{{ Table.query.filter_by(condition).count() }}

奖励积分(对于注重表现的人):

Bonus points (for the performance minded):

.count()有点慢(尤其是在MySQL中，由于对子查询的处理不善)，因此，您可以使用如下所示的自定义计数:

.count() is a bit slow (especially with MySQL, thanks to a poor handling of subqueries), so instead, you can use a custom count that looks like this:

db.session.execute(Table.query.filter_by(condition).statement.with_only_columns([func.count()]).order_by(None)).scalar()

假定 db 是您的SQLAlchemy实例(即 db = SQLAlchemy(app)).这很麻烦，但是可以节省一些大查询的开销.

That's assuming db is your SQLAlchemy instance (ie, db = SQLAlchemy(app)). It's a mouthful, but it will save you a little bit of overhead on big queries.

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