MongoDB检索特殊sports字段文档及单值转数组更新问题

Hey there! Let's tackle your two MongoDB questions one by one, with clear examples you can use right away.

1. 检索符合三种sports属性情况的文档 #

You need to target documents where sports is either an empty array, null, or doesn't exist at all. We can use the $or operator to combine these three conditions into a single query:

db.students.find({
  $or: [
    { sports: { $exists: false } }, // 匹配sports字段不存在的文档
    { sports: null }, // 匹配sports为null的文档
    { sports: { $size: 0 } } // 匹配sports为空数组的文档
  ]
})

小提示： #

If you want to return only specific fields (instead of the full document), add a projection parameter at the end, like:

db.students.find(
  { /* 上述条件 */ },
  { name: 1, _id: 0 } // 只返回name字段，不返回_id
)

2. 将单值sports转为数组并添加新运动 #

For documents where sports is a single string (e.g., sports: "Badminton"), we'll use MongoDB's pipeline-based update (available in 4.2+) to convert the string to an array and append your new sport. Here's how:

示例：添加"Tennis"到单值sports字段 #

db.students.updateMany(
  { sports: { $type: "string" } }, // 匹配所有sports为字符串类型的文档
  [
    {
      $set: {
        sports: {
          $concatArrays: [
            [ "$sports" ], // 将原字符串转为单元素数组
            [ "Tennis" ]   // 要添加的新运动（可以替换成你需要的内容）
          ]
        }
      }
    }
  ]
)

解释： #

• { $type: "string" } ensures we only target documents where sports is a single string value, not arrays or nulls.
• The pipeline update lets us reference the existing sports value directly. We wrap it in [ "$sports" ] to turn it into an array, then use $concatArrays to merge it with the new sport array.
• After running this, a document with sports: "Badminton" will become sports: [ "Badminton", "Tennis" ].

If you want to add multiple sports at once, just expand the second array: [ "Tennis", "Swimming" ].

内容的提问来源于stack exchange，提问作者Ernesto_Che