$（位置更新操作符）

更新插入

请勿在更新或插入（upsert）操作中使用 $操作符。如果更新查询与任何现有文档都不匹配，则更新或插入（upsert）将失败，因为 $操作符需要匹配的大量元素。

嵌套数组

You cannot use the positional $ operator for queries that traverse more than one array, such as queries that traverse arrays nested within other arrays, because the replacement for the $ placeholder is a single value.

取消设置

When you use the positional $ operator with the $unset operator, the positional operator does not remove the matching element from the array. Instead, it sets the element to null.

否定

如果查询使用否定操作符（例如 $ne、$not 或 $nin）匹配数组，则不能使用位置操作符更新该数组中的值。

You can use the positional operator if the negated portion of the query is inside an $elemMatch expression.

多个数组匹配

The positional $ update operator behaves ambiguously when you filter on multiple array fields.

When the server executes an update method, it first runs a query to determine which documents to update. If the update filters documents on multiple array fields, the positional $ update operator might not update the correct position in the array.

更多信息，请参阅示例。

更新数组中的值

Create the students collection with the following documents:

db.students.insertMany( [
   { "_id" : 1, "grades" : [ 85, 80, 80 ] },
   { "_id" : 2, "grades" : [ 88, 90, 92 ] },
   { "_id" : 3, "grades" : [ 85, 100, 90 ] }
] )

要将 grades 数组中第一个值为 80 的元素更新为 82，在不知道该元素在数组中的位置时，可使用位置 $ 操作符：

db.students.updateOne(
   { _id: 1, grades: 80 },
   { $set: { "grades.$" : 82 } }
)

位置 $ 操作符充当更新查询文档的第一个匹配项的占位符。

操作完成后，students 集合包含以下文档：

db.students.insertMany ( [
   { _id : 1, "grades" : [ 85, 82, 80 ] },
   { _id : 2, "grades" : [ 88, 90, 92 ] },
   { _id : 3, "grades" : [ 85, 100, 90 ] }
] )

更新数组中的文档

The positional $ operator allows updates to arrays that have embedded documents. Use the positional $ operator to access fields in embedded documents. Use dot notation on the $ operator.

db.collection.updateOne(
   { <query selector> },
   { <update operator>: { "array.$.field" : value } }
)

考虑下面 students 集合中的文档，其 grades 元素值是一个嵌入文档的数组：

db.students.insertOne( [
   {
      _id: 4,
      grades: [
         { grade: 80, mean: 75, std: 8 },
         { grade: 85, mean: 90, std: 5 },
         { grade: 85, mean: 85, std: 8 }
      ]
   }
] )

使用位置 $ 运算符来更新与“grade 等于 85 条件”相匹配的第一个数组元素的 std 字段：

db.students.updateOne(
   { _id: 4, "grades.grade": 85 },
   { $set: { "grades.$.std" : 6 } }
)

操作完成后，文档更新后的值如下：

{
   "_id" : 4,
   "grades" : [
      { "grade" : 80, "mean" : 75, "std" : 8 },
      { "grade" : 85, "mean" : 90, "std" : 6 },
      { "grade" : 85, "mean" : 85, "std" : 8 }
   ]
}

使用多字段匹配项更新嵌入文档

Use the $ operator to update the first array element that matches multiple query criteria specified with the $elemMatch operator.

以 students 集合中的以下文档为例；其中，grades 字段值为嵌入式文档数组：

db.students.insertOne( [
   {
     _id: 5,
      grades: [
         { grade: 80, mean: 75, std: 8 },
         { grade: 85, mean: 90, std: 5 },
         { grade: 90, mean: 85, std: 3 }
      ]
   }
] )

In the following example, the $ operator updates the value of the std field in the first embedded document that has a grade field with a value less than or equal to 90 and a mean field with a value greater than 80:

db.students.updateOne(
   {
     _id: 5,
     grades: { $elemMatch: { grade: { $lte: 90 }, mean: { $gt: 80 } } }
   },
   { $set: { "grades.$.std" : 6 } }
)

此操作会更新与条件匹配的第一个嵌入式文档，即数组中的第二个嵌入式文档：

{
  _id: 5,
  grades: [
    { grade: 80, mean: 75, std: 8 },
    { grade: 85, mean: 90, std: 6 },
    { grade: 90, mean: 85, std: 3 }
  ]
}

用多个数组匹配更新

当查询有多个数组字段用于过滤集合中的文档时，位置 $ 更新操作符就会表现得不明确。

以 students_deans_list 集合中的某一文档为例，其中包含学生信息数组：

db.students_deans_list.insertMany( [
   {
      _id: 8,
      activity_ids: [ 1, 2 ],
      grades: [ 90, 95 ],
      deans_list: [ 2021, 2020 ]
   }
] )

In the following example, the user attempts to modify the deans_list field. The example filters documents using the activity_ids, deans_list, and grades fields, and updates the 2021 value in the deans_list field to 2022:

db.students_deans_list.updateOne(
   { activity_ids: 1, grades: 95, deans_list: 2021 },
   { $set: { "deans_list.$": 2022 } }
)

When the server executes the preceding updateOne method, it filters the available documents using values in the supplied array fields. Although the deans_list field is used in the filter, it is not the field used by the positional $ update operator to determine which position in the array to update:

db.students_deans_list.find( { _id: 8 } )

示例输出：

{
   _id: 8,
   activity_ids: [ 1, 2 ],
   grades: [ 90, 95 ],
   deans_list: [ 2021, 2022 ]
}

updateOne 方法与 2021 上的 deans_list 字段匹配，但位置 $ 更新操作符却将 2020 值改为 2022。

To avoid unexpected results when matching on multiple arrays, use the filtered positional operator $[<identifier>].