将第一个数组值复制到 MongoDB 中的另一个字段 [英] Copy first array value to another field in MongoDB

问题描述

我有一个旧的产品列表，将描述存储在索引 [0] 处的数组中.该模型设置为字符串.所以我的计划是提取该值并将其添加到临时字段中.第 2 步是获取该值并将其复制到原始字段.这是我想要修复的错误"产品.

I have an old list of products that store the descriptions in an array at index [0]. The model is set up as a string. So my plan is to extract that value and add it to a temporary field. Step 2 would be to take that value and copy it to the original field. This is the 'wrong' product I want to fix.

{_id : 1 , pDescription : ['great product']},
{_id : 2 , pDescription : ['another product']}

我只想将数组更改为这样的字符串:

All I want to is to change the array to a string like this:

{_id : 1 , pDescription : 'great product'},
{_id : 2 , pDescription : 'another product'}

我试过这个来创建临时描述:

I have tried this to create the temporary description:

    Products.aggregate([
    {
        $match: {
            pDescription: {
                $type: "array"
            }
        }
        },
    {
        $set: {
            pDescTemp: {
                $first: "$pDescription"
            }
        }
            }
        ]).exec((err, r) => {
       // do stuff here
   });

该命令在没有 $first 命令的情况下工作正常.错误内容为:MongoError: Unrecognized expression '$first'

The command works fine without the $first command. The error reads: MongoError: Unrecognized expression '$first'

感谢您提供有关如何解决此问题的任何提示！谢谢！

Any tips on how to fix this are appreciated! Thanks!

推荐答案

我相信这是您需要更新 pDescription 字段以等于已存储为 <代码>p描述:

I believe this is what you need to update your pDescription field to be equal to the first element of the array already stored as pDescription:

db.Products.updateMany({},
[
  {
    $set: {
      pDescription: {
        $arrayElemAt: [
          "$pDescription",
          0
        ]
      }
    }
  }
])

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