MongoDB:聚合框架:字段之间的$ match [英] MongoDB : aggregation framework : $match between fields

问题描述

我有一个包含两个文档的测试集合:

I have a test collection with two documents :

> db.test.find().pretty()
{ "_id" : ObjectId("510114b46c1a3a0f6e5dd7aa"), "a" : 1, "b" : 2 }
{ "_id" : ObjectId("510114c86c1a3a0f6e5dd7ab"), "a" : 3, "b" : 1 }

使用聚合框架，我只想获取a大于b的文档. $ gt仅获取参数中的值，而不获取字段中的值...

With aggregation framework, I want to get only the documents where a is greater than b. $gt get only values in argument not fields...

> db.test.aggregate([{"$match":{"$a":{"$gt":"$b"}}}])
{ "result" : [ ], "ok" : 1 } /* don't work*/

您有什么想法吗?

预先感谢

最诚挚的问候

推荐答案

嗯，我没有做太多测试，我会说你可以使用$cmp来实现这一目的:

Hmm without much testing on my end I will say you can use $cmp for this:

http://docs.mongodb.org/manual/reference/aggregation /cmp/#_ S_cmp

db.test.aggregate([
    {$project: {
        // All your other fields here
        cmp_value: {$cmp: ['$a', '$b']}
    }},
    {$match: {cmp_value: {$gt: 0}}} 
])

也许有更好的方法，但是我附近没有MongoDB安装可以测试.

There might be a better way but I haven't got a MongoDB installation near me to test.

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