$addFields (aggregation stage)（聚合阶段）

Using Two $addFields Stages使用两个$addFields阶段

A collection called scores contains the following documents:名为scores的集合包含以下文档：

db.scores.insertMany( [
   {
      _id: 1,
      student: "Maya",
      homework: [ 10, 5, 10 ],
      quiz: [ 10, 8 ],
      extraCredit: 0
   },
   {
      _id: 2,
      student: "Ryan",
      homework: [ 5, 6, 5 ],
      quiz: [ 8, 8 ],
      extraCredit: 8
   }
] )

The following operation uses two $addFields stages to include three new fields in the output documents:以下操作使用两个$addFields阶段在输出文档中包含三个新字段：

db.scores.aggregate( [
   {
   $addFields: {
      totalHomework: { $sum: "$homework" } ,
      totalQuiz: { $sum: "$quiz" }
   }
   },
   {
   $addFields: { totalScore:
      { $add: [ "$totalHomework", "$totalQuiz", "$extraCredit" ] } }
   }
] )

The operation returns the following documents:该操作返回以下文档：

[
   {
      _id: 1,
      student: "Maya",
      homework: [ 10, 5, 10 ],
      quiz: [ 10, 8 ],
      extraCredit: 0,
      totalHomework: 25,
      totalQuiz: 18,
      totalScore: 43
   },
   {
      _id: 2,
      student: "Ryan",
      homework: [ 5, 6, 5 ],
      quiz: [ 8, 8 ],
      extraCredit: 8,
      totalHomework: 16,
      totalQuiz: 16,
      totalScore: 40
   }
]

Adding Fields to an Embedded Document向嵌入式文档添加字段

Use dot notation to add new fields to embedded documents.使用点符号向嵌入式文档添加新字段。

For example, create a collection called vehicles with the following documents:例如，使用以下文档创建一个名为vehicles的集合：

db.vehicles.insertMany( [
      { _id: 1, type: "car", specs: { doors: 4, wheels: 4 } },
      { _id: 2, type: "motorcycle", specs: { doors: 0, wheels: 2 } },
      { _id: 3, type: "jet ski" }
] )

The following aggregation operation adds a new field fuel_type to the embedded document specs.以下聚合操作将一个新的字段fuel_type添加到嵌入式文档specs中。

db.vehicles.aggregate( [
   { $addFields: { "specs.fuel_type": "unleaded" } }
] )

The operation returns the following results:该操作返回以下结果：

[
   { _id: 1, type: "car",
      specs: { doors: 4, wheels: 4, fuel_type: "unleaded" } },
   { _id: 2, type: "motorcycle",
      specs: { doors: 0, wheels: 2, fuel_type: "unleaded" } },
   { _id: 3, type: "jet ski",
      specs: { fuel_type: "unleaded" } }
]

Overwriting an existing field覆盖现有字段

Specifying an existing field name in an $addFields operation causes the original field to be replaced.在$addFields操作中指定现有字段名会导致替换原始字段。

A collection called animals contains the following document:一个名为animals的集合包含以下文件：

db.animals.insertOne(
   { _id: 1, dogs: 10, cats: 15 }
)

The following $addFields operation specifies the cats field.以下$addFields操作指定了cats字段。

db.animals.aggregate( [
{
   $addFields: { cats: 20 }
}
] )

The operation returns the following document:该操作返回以下文档：

[ { _id: 1, dogs: 10, cats: 20 } ]

db.fruit.insertMany( [
   { _id: 1, item: "tangerine", type: "citrus" },
   { _id: 2, item: "lemon", type: "citrus" },
   { _id: 3, item: "grapefruit", type: "citrus" }
] )

db.fruit.aggregate( [
{
   $addFields: {
      _id : "$item",
      item: "fruit"
   }
}
] )

[
   { _id: "tangerine", item: "fruit", type: "citrus" },
   { _id: "lemon", item: "fruit", type: "citrus" },
   { _id: "grapefruit", item: "fruit", type: "citrus" }
]

Add Element to an Array向数组添加元素

Create a sample scores collection with the following:使用以下内容创建样本scores集合：

db.scores.insertMany( [
   { _id: 1, student: "Maya", homework: [ 10, 5, 10 ], quiz: [ 10, 8 ], extraCredit: 0 },
   { _id: 2, student: "Ryan", homework: [ 5, 6, 5 ], quiz: [ 8, 8 ], extraCredit: 8 }
] )

You can use $addFields with a $concatArrays expression to add an element to an existing array field. 您可以将$addFields与$concatArrays表达式一起使用，向现有数组字段添加元素。For example, the following operation uses $addFields to replace the homework field with a new array whose elements are the current homework array concatenated with another array containing a new score [ 7 ].例如，以下操作使用$addFields将homework字段替换为一个新数组，该数组的元素是当前homework数组与包含新分数的另一个数组连接起来的元素[7]。

db.scores.aggregate( [
   { $match: { _id: 1 } },
   { $addFields: { homework: { $concatArrays: [ "$homework", [ 7 ] ] } } }
] )

The operation returns the following:该操作返回以下内容：

[ { _id: 1, student: "Maya", homework: [ 10, 5, 10, 7 ], quiz: [ 10, 8 ], extraCredit: 0 } ]

Remove Fields删除字段

You can use $addFields with the $$REMOVE variable to remove document fields.您可以使用$addFields和$$REMOVE变量来删除文档字段。

For example, create a labReadings collection:例如，创建一个labReadings集合：

db.labReadings.insertMany( [
   {
      date: ISODate("2024-10-09"),
      temperature: 80
   },
   {
      date: null,
      temperature: 83
   },
   {
      date: ISODate("2024-12-09"),
      temperature: 85
   }
] )

To remove the date field from the labReadings documents, use $addFields with the $$REMOVE variable:要从labReadings文档中删除日期字段，请使用$addFields和$$remove变量：

db.labReadings.aggregate( [
   {
      $addFields: { date: "$$REMOVE" }
   }
] )

Output:输出

[
   { _id: ObjectId('671285306fd2c3b24f2e7eaa'), temperature: 80 },
   { _id: ObjectId('671285306fd2c3b24f2e7eab'), temperature: 83 },
   { _id: ObjectId('671285306fd2c3b24f2e7eac'), temperature: 85 }
]

db.labReadings.aggregate( [
   {
      $addFields:
         {
            date: {
               $ifNull: [ "$date", "$$REMOVE" ]
            }
         }
   }
] )

[
   {
      _id: ObjectId('671285306fd2c3b24f2e7eaa'),
      date: ISODate('2024-10-09T00:00:00.000Z'),
      temperature: 80
   },
   { _id: ObjectId('671285306fd2c3b24f2e7eab'), temperature: 83 },
   {
      _id: ObjectId('671285306fd2c3b24f2e7eac'),
      date: ISODate('2024-12-09T00:00:00.000Z'),
      temperature: 85
   }
]