如何将来自多个MongoDB记录的字符串结果串联到MongoDB中的单个结果中? [英] How to concatenate string results from multiple MongoDB records into a single result in MongoDB?

问题描述

假定以下记录:

{ text: "foo"},
{ text: "bar"}

如何获得如下结果:

results: "foo bar"

我最接近的是使用 $ addToSet ，但这只会创建一个带有结果的数组，而不是我真正想要的单个字符串.

The closest I am getting to it is to use $addToSet but this just creates an array with the results instead of a single string which is what I really want.

results: [ "foo", "bar" ] 

使用Mongo 3.4

Using Mongo 3.4

推荐答案

使用 $ reduce 与 $concat 获得一个字符串:

Use $group to get an array from all the documents and then $reduce with $concat to get one string:

db.col.aggregate([
    {
        $group: {
            _id: null,
            text: { $push: "$text" }
        }
    },
    {
        $project: {
            text: {
                $reduce: {
                    input: "$text",
                    initialValue: "",
                    in: {
                        $cond: [ { "$eq": [ "$$value", "" ] }, "$$this", { $concat: [ "$$value", " ", "$$this" ] } ]
                    }
                }
            }
        }
    }
])

在 $ group 之后，您将获得一个包含所有 text 值的数组的单个文档.然后 $ reduce 扫描"数组并将状态( $$ value )与当前处理的项目连接在一起.对于第一个项目，状态将是一个空字符串，因此我在使用 $ cond 以避免开头出现空格.

After $group you will get single document which contains an array of all text values. Then $reduce "scans" the array and concatenates the state ($$value) with currently processed item. For the first item state will be an empty string so I'm using $cond to avoid having whitespace on the beginning.

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