MongoDB:$将计算表达式推入$ group中 [英] MongoDB: $push a computed expression in a $group

问题描述

作为$group操作的一部分，我想将计算的表达式推到数组上，而不是将值本身推到数组上. MongoDB可以做到这一点吗?

I'd like to push a computed expression onto an array instead of a value itself as part of a $group operation. Can MongoDB do this?

例如，如果我有一个名为c的集合，其中包含这样的文档:

For example, if I have a collection named c with documents like this:

{ "k" : 1, "v" : true }
{ "k" : 1, "v" : true }
{ "k" : 1, "v" : false }
{ "k" : 1, "v" : true }
{ "k" : 1, "v" : false }

然后我可以像这样进行聚合，以获取数组中的所有v值:

Then I can do an aggregation like this to get all v values in an array:

db.c.aggregate({$group: {_id:'$k', vals: {$push: '$v'}}})

{
    "result" : [
        {
            "_id" : 1,
            "vals" : [
                true,
                true,
                false,
                true,
                false
            ]
        }
    ],
    "ok" : 1
}

但是，如果我想将它们更改为'red'和'green'，而不是实际的true和false值，该怎么办?也许像这样:

But what if instead of the actual true and false values, I'd like to change them to, say, 'red' and 'green'? Perhaps something like:

db.c.aggregate({$group: {_id:'$k', vals: {$push: '$v' ? 'red' : 'green'}}})

但是，这只是输出所有'red'.可以在$push表达式中计算任意值吗?

But alas, this just outputs all 'red'. Is it possible to compute arbitrary values in a $push expression?

推荐答案

您可以使用$cond(

编辑

正如所提到的另一种编辑，实际上它需要$eq才能工作，因为{'$v':true}并未作为正确的表达式被评估.

Edit

As another edit mentioned, it actually needs $eq to work since {'$v':true} isn't evaulated as a correct expression.

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