Mongoid:如何查询所有值为nil的对象? [英] Mongoid: how to query for all objects where value is nil?

问题描述

我在做诸如以下的事情时遇到了困难:

I'm having a difficult time doing something such as:

Something.where(:field => nil) 

或

Something.where(:field => { '$eq' => nil })

在Mongoid中处理此问题的正确方法是什么?

What's the right way to handle this in Mongoid?

推荐答案

这是正确的方法.例如，要查找引擎为nil的汽车，请使用:

That's the right way to do it. To find cars whose engine is nil, for example, use:

# Cars that have a _nil_ engine.
Car.where(:engine => nil)

如果您要查找字段的缺少(而不是设置为nil的字段)，请使用$exists谓词:

If you're trying to look for the absence of a field (rather than one that's set to nil), use the $exists predicate:

# Cars that lack an engine entirely.
Car.where(:engine.exists => false)

请注意，将字段foo设置为nil并缺少名为foo的字段是两件事.

Note that setting a field foo to be nil and lacking a field named foo are two different things.

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