$addFields (aggregation)

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：

{
  _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括三个新字段：

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

The operation returns the following documents:操作将返回以下文档：

{
  "_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向嵌入文档添加字段

Use dot notation to add new fields to embedded documents. 使用点符号向嵌入文档添加新字段。A collection called vehicles contains the following documents:一个名为vehicles的集合包含以下文档：

{ _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.以下聚合操作将新字段fuel_type添加到嵌入文档规范中。

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的集合包含以下文档：

{ _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:运算返回以下文档：

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

It is possible to replace one field with another. 可以用另一个字段替换一个字段。In the following example the item field substitutes for the _id field.在以下示例中，item字段替换_id字段。

A collection called fruit contains the following documents:一个名为fruit的集合包含以下文档：

{ "_id" : 1, "item" : "tangerine", "type" : "citrus" }
{ "_id" : 2, "item" : "lemon", "type" : "citrus" }
{ "_id" : 3, "item" : "grapefruit", "type" : "citrus" }

The following aggregration operation uses $addFields to replace the _id field of each document with the value of the item field, and replaces the item field with a static value.以下聚合操作使用$addFields将每个文档的_id字段替换为item字段的值，并将item字段替换为静态值。

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

The operation returns the following:运算结果如下：

{ "_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:创建具有以下内容的示例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 $concatArrays expression to add an element to an existing array field. 可以使用$addFields和$concatArrays表达式将元素添加到现有数组字段。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[7]的另一个数组串联。

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

The operation returns the following:运算结果如下：

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