Updates with Aggregation Pipeline使用聚合管道的更新

updateOne with 配合$set

Create an example students collection (if the collection does not currently exist, insert operations will create the collection):创建示例students集合（如果该集合当前不存在，插入操作将创建该集合）：

db.students.insertMany( [
   { _id: 1, test1: 95, test2: 92, test3: 90, modified: new Date("01/05/2020") },
   { _id: 2, test1: 98, test2: 100, test3: 102, modified: new Date("01/05/2020") },
   { _id: 3, test1: 95, test2: 110, modified: new Date("01/04/2020") }
] )

To verify, query the collection:要验证，请查询集合：

db.students.find()

The following db.collection.updateOne() operation uses an aggregation pipeline to update the document with _id: 3:以下db.collection.updateOne()操作使用聚合管道更新_id: 3的文档：

db.students.updateOne( { _id: 3 }, [ { $set: { "test3": 98, modified: "$$NOW"} } ] )

Specifically, the pipeline consists of a $set stage which adds the test3 field (and sets its value to 98) to the document and sets the modified field to the current datetime. 具体来说，管道由一个$set阶段组成，该阶段将test3字段（并将其值设置为98）添加到文档中，并将modified字段设置为当前日期时间。The operation uses the aggregation variable NOW for the current datetime. 该操作使用聚合变量NOW作为当前日期时间。To access the variable, prefix with $$ and enclose in quotes.要访问该变量，请在前面加上$$并将其括在引号中。

To verify the update, you can query the collection:要验证更新，您可以查询集合：

db.students.find().pretty()

updateMany with 配合$replaceRoot and 和$set

Create an example students2 collection (if the collection does not currently exist, insert operations will create the collection):创建示例students2集合（如果该集合当前不存在，插入操作将创建该集合）：

db.students2.insertMany( [
   { "_id" : 1, quiz1: 8, test2: 100, quiz2: 9, modified: new Date("01/05/2020") },
   { "_id" : 2, quiz2: 5, test1: 80, test2: 89, modified: new Date("01/05/2020") },
] )

To verify, query the collection:要验证，请查询集合：

db.students2.find()

The following db.collection.updateMany() operation uses an aggregation pipeline to standardize the fields for the documents (i.e. documents in the collection should have the same fields) and update the modified field:以下db.collection.updateMany()操作使用聚合管道来标准化文档的字段（即集合中的文档应具有相同的字段），并更新modified字段：

db.students2.updateMany( {},
  [
    { $replaceRoot: { newRoot:
       { $mergeObjects: [ { quiz1: 0, quiz2: 0, test1: 0, test2: 0 }, "$$ROOT" ] }
    } },
    { $set: { modified: "$$NOW"}  }
  ]
)

Specifically, the pipeline consists of:具体而言，该管道由以下部分组成：

• a $replaceRoot stage with a $mergeObjects expression to set default values for the quiz1, quiz2, test1 and test2 fields. 一个$replaceRoot阶段，配合$mergeObjects表达式，用于为quiz1、quiz2、test1和test2字段设置默认值。The aggregation variable ROOT refers to the current document being modified. To access the variable, prefix with $$ and enclose in quotes. The current document fields will override the default values.聚合变量ROOT是指正在修改的当前文档。要访问该变量，请在前面加上$$并将其括在引号中。当前文档字段将覆盖默认值。
• a $set stage to update the modified field to the current datetime. The operation uses the aggregation variable NOW for the current datetime. To access the variable, prefix with $$ and enclose in quotes.$set阶段，用于将modified字段更新为当前日期时间。该操作使用聚合变量NOW作为当前日期时间。要访问该变量，请在前面加上$$并将其括在引号中。

To verify the update, you can query the collection:要验证更新，您可以查询集合：

db.students2.find()

updateMany with 配合$set

Create an example students3 collection (if the collection does not currently exist, insert operations will create the collection):创建一个示例students3集合（如果该集合当前不存在，插入操作将创建该集合）：

db.students3.insertMany( [
   { "_id" : 1, "tests" : [ 95, 92, 90 ], "modified" : ISODate("2019-01-01T00:00:00Z") },
   { "_id" : 2, "tests" : [ 94, 88, 90 ], "modified" : ISODate("2019-01-01T00:00:00Z") },
   { "_id" : 3, "tests" : [ 70, 75, 82 ], "modified" : ISODate("2019-01-01T00:00:00Z") }
] );

To verify, query the collection:要验证，请查询集合：

db.students3.find()

The following db.collection.updateMany() operation uses an aggregation pipeline to update the documents with the calculated grade average and letter grade.以下db.collection.updateMany()操作使用聚合管道使用计算出的平均成绩和字母成绩更新文档。

db.students3.updateMany(
   { },
   [
     { $set: { average : { $trunc: [ { $avg: "$tests" }, 0 ] }, modified: "$$NOW" } },
     { $set: { grade: { $switch: {
                           branches: [
                               { case: { $gte: [ "$average", 90 ] }, then: "A" },
                               { case: { $gte: [ "$average", 80 ] }, then: "B" },
                               { case: { $gte: [ "$average", 70 ] }, then: "C" },
                               { case: { $gte: [ "$average", 60 ] }, then: "D" }
                           ],
                           default: "F"
     } } } }
   ]
)

Specifically, the pipeline consists of:具体而言，该管道由以下部分组成：

• a $set stage to calculate the truncated average value of the tests array elements and to update the modified field to the current datetime. $set阶段，用于计算tests数组元素的截断平均值，并将modified字段更新为当前日期时间。To calculate the truncated average, the stage uses the $avg and $trunc expressions. 为了计算截断的平均值，该阶段使用$avg和$trunc表达式。The operation uses the aggregation variable NOW for the current datetime. 该操作使用聚合变量NOW作为当前日期时间。To access the variable, prefix with $$ and enclose in quotes.要访问该变量，请在前面加上$$并将其括在引号中。
• a $set stage to add the grade field based on the average using the $switch expression.$set阶段，使用$switch表达式根据average（平均值）添加grade字段。

To verify the update, you can query the collection:要验证更新，您可以查询集合：

db.students3.find()

updateOne with 配合$set

Create an example students4 collection (if the collection does not currently exist, insert operations will create the collection):创建示例students4集合（如果该集合当前不存在，插入操作将创建该集合）：

db.students4.insertMany( [
  { "_id" : 1, "quizzes" : [ 4, 6, 7 ] },
  { "_id" : 2, "quizzes" : [ 5 ] },
  { "_id" : 3, "quizzes" : [ 10, 10, 10 ] }
] )

To verify, query the collection:要验证，请查询集合：

db.students4.find()

The following db.collection.updateOne() operation uses an aggregation pipeline to add quiz scores to the document with _id: 2:以下db.collection.updateOne()操作使用聚合管道将测验分数添加到_id: 2的文档中：

db.students4.updateOne( { _id: 2 },
  [ { $set: { quizzes: { $concatArrays: [ "$quizzes", [ 8, 6 ]  ] } } } ]
)

To verify the update, query the collection:要验证更新，请查询集合：

db.students4.find()

updateMany with 配合$addFields

Create an example temperatures collection that contains temperatures in Celsius (if the collection does not currently exist, insert operations will create the collection):创建一个包含摄氏温度的示例temperatures集合（如果该集合当前不存在，插入操作将创建该集合）：

db.temperatures.insertMany( [
  { "_id" : 1, "date" : ISODate("2019-06-23"), "tempsC" : [ 4, 12, 17 ] },
  { "_id" : 2, "date" : ISODate("2019-07-07"), "tempsC" : [ 14, 24, 11 ] },
  { "_id" : 3, "date" : ISODate("2019-10-30"), "tempsC" : [ 18, 6, 8 ] }
] )

To verify, query the collection:要验证，请查询集合：

db.temperatures.find()

The following db.collection.updateMany() operation uses an aggregation pipeline to update the documents with the corresponding temperatures in Fahrenheit:以下db.collection.updateMany()操作使用聚合管道将文档更新为相应的华氏温度：

db.temperatures.updateMany( { },
  [
    { $addFields: { "tempsF": {
          $map: {
             input: "$tempsC",
             as: "celsius",
             in: { $add: [ { $multiply: ["$$celsius", 9/5 ] }, 32 ] }
          }
    } } }
  ]
)

Specifically, the pipeline consists of an $addFields stage to add a new array field tempsF that contains the temperatures in Fahrenheit. 具体来说，该管道包含一个$addFields阶段，用于添加一个新的数组字段tempsF，其中包含华氏温度。To convert each celsius temperature in the tempsC array to Fahrenheit, the stage uses the $map expression with $add and $multiply expressions.要将tempsC数组中的每个摄氏温度转换为华氏温度，该阶段使用$map表达式以及$add和$multiply表达式。

To verify the update, you can query the collection:要验证更新，您可以查询集合：

db.temperatures.find()

Update with let Variables使用let变量进行更新

To define variables that you can access elsewhere in the command, use the let option.要定义可以在命令的其他地方访问的变量，请使用let选项。

Create a collection cakeFlavors:创建一个系列cakeFlavors：

db.cakeFlavors.insertMany( [
   { _id: 1, flavor: "chocolate" },
   { _id: 2, flavor: "strawberry" },
   { _id: 3, flavor: "cherry" }
] )

The following updateOne command uses variables set with the let option:以下updateOne命令使用用let选项设置的变量：

• The targetFlavor variable is set to cherry. This variable is used in the $eq expression to specify the match filter.targetFlavor变量设置为cherry。此变量在$eq表达式中用于指定匹配筛选器。
• The newFlavor variable is set to orange. This variable is used in the $set operator to specify the updated flavor value for the matched document.newFlavor变量设置为orange。此变量在$set运算符中用于指定匹配文档的更新flavor值。

db.cakeFlavors.updateOne(
   {
      $expr: { $eq: [ "$flavor", "$$targetFlavor" ] }
   },
   [
      {
         $set: { flavor: "$$newFlavor" }
      }
   ],
   {
      let: { targetFlavor: "cherry", newFlavor: "orange" }
   }
)

After you run the preceding update operation, the cakeFlavors collection contains these documents:运行上述更新操作后，cakeLavors集合包含以下文档：

[
   { _id: 1, flavor: 'chocolate' },
   { _id: 2, flavor: 'strawberry' },
   { _id: 3, flavor: 'orange' }
]

Additional Examples其他示例

See also the various update method pages for additional examples:其他示例请参见各种更新方法页面：

• db.collection.updateOne
• db.collection.updateMany
• db.collection.findOneAndUpdate()
• db.collection.findAndModify()
• Bulk.find.update()
• Bulk.find.updateOne()
• Bulk.find.upsert()