如果它是一个数组，如何连接字符串数组字段以显示为一个字符串

Question

以下是我collection中的两个文档：

{
    "_id": {
        "$oid": "5f48e358d43721376c397f54"
    },
    "heading": "this is heading",
    "tags": ["tag1","tag2","tag3"],
    "categories": ["projA", "projectA2"],
    "content": ["This", "is", "the", "content", "of", "the", "document"],
    "timestamp": 1598612312.506219,
    "lang": "en"
}

和

{
    "_id": {
        "$oid": "5f48e358d43721376c397f53"
    },
    "heading": "this is heading",
    "tags": "tag1,tag2,tag3",
    "categories": "projA, projectA2",
    "content": "This is the content of the document",
    "timestamp": 1598612312.506219,
    "lang": "en"
}

我想编写一个查询，连接第一个文档的 content 元素并按原样打印第二个（因为它不是数组）。

我正在使用以下脚本来连接，但它在第二个文档上给出了错误，说它需要是一个数组，但事实并非如此。我不知道如何在这种情况下使用 switch 语句或嵌套 if 条件：

    "$addFields": {
  content: { 
      "$reduce": {
        "input": "$content",
        "initialValue": "",
        "in": {
          "$cond": {
            "if": { "$eq": [ { "$indexOfArray": [ "$content", "$$this" ] }, 0 ] },
            "then": { "$concat": [ "$$value", "$$this" ] },
            "else": { "$concat": [ "$$value", "\n", "$$this" ] }
          }    
        }
      }        
    }
}

我是 mongo 数据库的新手，因此我们将不胜感激。

Answer 1

你可以试试，

$cond 检查内容类型是否为数组，$reduce 连接内容字符串，$trim 删除白色 space

db.collection.aggregate([
  {
    $addFields: {
      content: {
        $cond: [
          { $eq: [{ $type: "$content" }, "array"] },
          {
            $trim: {
              input: {
                $reduce: {
                  input: "$content",
                  initialValue: "",
                  in: { $concat: ["$$value", " ", "$$this"] }
                }
              }
            }
          },
          "$content"
        ]
      }
    }
  }
])

Playground

如果它是一个数组，如何连接字符串数组字段以显示为一个字符串

How to concatenate string array field to show as one string if its an array

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