$[]

upsert

If an upsert operation results in an insert, the query must include an exact equality match on the array field in order to use the $[] positional operator in the update statement.如果upsert操作导致插入，则query必须在数组字段上包含完全相等的匹配项，才能在更新语句中使用$[]位置运算符。

For example, the following upsert operation, which uses $[] in the update document, specifies an exact equality match condition on the array field:例如，以下upsert操作在更新文档中使用$[]，在数组字段上指定完全相等的匹配条件：

db.collection.updateOne(
   { myArray: [ 5, 8 ] },
   { $set: { "myArray.$[]": 10 } },
   { upsert: true }
)

If no such document exists, the operation would result in an insertion of the following document:如果不存在此类文件，操作将导致插入以下文件：

{ "_id" : ObjectId(...), "myArray" : [ 10, 10 ] }

If the upsert operation did not include an exact equality match and no matching documents were found to update, the upsert operation would error.如果upsert操作不包括完全相等的匹配项，并且找不到要更新的匹配文档，则upsert操作将出错。

For example, the following operations would error if no matching documents were found to update:例如，如果找不到要更新的匹配文档，则以下操作将出错：

db.emptyCollection.updateOne(
   { },
   { $set: { "myArray.$[]": 10 } },
   { upsert: true }
)

db.emptyCollection.updateOne(
   { myArray: 5 },
   { $set: { "myArray.$[]": 10 } },
   { upsert: true }
)

Nested Arrays嵌套数组

The $[] operator can be used for queries that traverse more than one array and nested arrays.$[]运算符可用于遍历多个数组和嵌套数组的查询。

For an example, see Update Nested Arrays in Conjunction with $[<identifier>].有关示例，请参阅使用$[<identifier>]更新嵌套数组。

Update All Elements in an Array更新数组中的所有元素

Create the students collection:创建students集合：

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

To increment all elements in the grades array by 10 for all documents in the collection, use the all positional $[] operator:要将集合中所有文档的grades数组中的所有元素增加10，请使用所有位置$[]运算符：

db.students.updateMany(
   { },
   { $inc: { "grades.$[]": 10 } },
)

The all positional $[] operator acts as a placeholder for all elements in the array field.所有位置$[]运算符充当数组字段中所有元素的占位符。

After the operation, the students collection contains the following documents:操作后，students集合包含以下文档：

{ "_id" : 1, "grades" : [ 95, 92, 90 ] }
{ "_id" : 2, "grades" : [ 98, 100, 102 ] }
{ "_id" : 3, "grades" : [ 95, 110, 100 ] }

Update All Documents in an Array更新数组中的所有文档

The $[] positional operator facilitates updates to arrays that contain embedded documents. $[]位置运算符便于更新包含嵌入文档的数组。To access the fields in the embedded documents, use the dot notation with the $[] operator.若要访问嵌入文档中的字段，请使用带有$[]运算符的点表示法。

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

Create the students2 collection:创建students2集合：

db.students2.insertMany( [
   {
      "_id" : 1,
      "grades" : [
         { "grade" : 80, "mean" : 75, "std" : 8 },
         { "grade" : 85, "mean" : 90, "std" : 6 },
         { "grade" : 85, "mean" : 85, "std" : 8 }
      ]
   },
   {
      "_id" : 2,
      "grades" : [
         { "grade" : 90, "mean" : 75, "std" : 8 },
         { "grade" : 87, "mean" : 90, "std" : 5 },
         { "grade" : 85, "mean" : 85, "std" : 6 }
      ]
   }
] )

To modify the value of the std field for all elements in the grades array, use the positional $[] operator:要修改grades数组中所有元素的std字段的值，请使用位置$[]运算符：

db.students2.updateMany(
   { },
   { $inc: { "grades.$[].std" : -2 } },
)

After the operation, the collection has the following documents:操作后，集合具有以下文档：

{
   "_id" : 1,
   "grades" : [
      { "grade" : 80, "mean" : 75, "std" : 6 },
      { "grade" : 85, "mean" : 90, "std" : 4 },
      { "grade" : 85, "mean" : 85, "std" : 6 }
   ]
}
{
   "_id" : 2,
   "grades" : [
      { "grade" : 90, "mean" : 75, "std" : 6 },
      { "grade" : 87, "mean" : 90, "std" : 3 },
      { "grade" : 85, "mean" : 85, "std" : 4 }
   ]
}

Update Arrays Specified Using a Negation Query Operator更新使用否定查询运算符指定的数组

Create the results collection:创建results集合：

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

To increment all elements in the grades array by 10 for all documents except those with the value 100 in the grades array, use the all positional $[] operator:要将除grades数组中值为100的文档外的所有文档的等级数组中的所有元素增加10，请使用全部位置$[]运算符：

db.results.updateMany(
   { "grades" : { $ne: 100 } },
   { $inc: { "grades.$[]": 10 } },
)

The all positional $[] operator acts as a placeholder for all elements in the array field.全位置$[]运算符充当数组字段中所有元素的占位符。

After the operation, the students collection contains the following documents:操作后，students集合包含以下文档：

{ "_id" : 1, "grades" : [ 95, 92, 90 ] }
{ "_id" : 2, "grades" : [ 98, 100, 102 ] }
{ "_id" : 3, "grades" : [ 85, 100, 90 ] }

Update Nested Arrays in Conjunction with $[<identifier>]使用$[<identifier>]更新嵌套数组

The $[] positional operator, in conjunction with filter $[<identifier>] positional operator can be used to update nested arrays.$[]位置运算符与筛选器$[<identifier>]位置运算符可用于更新嵌套数组。

Create a collection students3 with the following documents:使用以下文档创建集合students3：

db.students3.insertMany( [
   { "_id" : 1,
      "grades" : [
        { type: "quiz", questions: [ 10, 8, 5 ] },
        { type: "quiz", questions: [ 8, 9, 6 ] },
        { type: "hw", questions: [ 5, 4, 3 ] },
        { type: "exam", questions: [ 25, 10, 23, 0 ] },
      ]
   }
] )

To update all values that are greater than or equal to 8 in the nested grades.questions array, regardless of type:要更新嵌套grades.questions数组中大于或等于8的所有值，无论type如何：

db.students3.updateMany(
   {},
   { $inc: { "grades.$[].questions.$[score]": 2 } },
   { arrayFilters: [  { "score": { $gte: 8 } } ] }
)

The updated documents look like this:更新后的文档如下所示：

{
  _id: 1,
  grades: [
    { type: 'quiz', questions: [ 12, 10, 5 ] },
    { type: 'quiz', questions: [ 10, 11, 6 ] },
    { type: 'hw', questions: [ 5, 4, 3 ] },
    { type: 'exam', questions: [ 27, 12, 25, 0 ] }
  ]
}