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$addFields (aggregation)

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Definition定义

$addFields

Adds new fields to documents. 向文档中添加新字段。$addFields outputs documents that contain all existing fields from the input documents and newly added fields.$addFields输出包含输入文档和新添加字段中所有现有字段的文档。

The $addFields stage is equivalent to a $project stage that explicitly specifies all existing fields in the input documents and adds the new fields.$addFields阶段相当于$project阶段，它显式指定输入文档中的所有现有字段并添加新字段。

Note

Starting in version 4.2, MongoDB adds a new aggregation pipeline stage $set that is an alias for $addFields.从4.2版本开始，MongoDB添加了一个新的聚合管道阶段$set，它是$addFields的别名。

Syntax语法

The stage has the following syntax:该阶段具有以下语法：

{ $addFields: { <newField>: <expression>, ... } }

Specify the name of each field to add and set its value to an aggregation expression or an empty object. For more information on expressions, see Expressions.指定要添加的每个字段的名称，并将其值设置为聚合表达式或空对象。有关表达式的详细信息，请参阅表达式。

Important

If the name of the new field is the same as an existing field name (including _id), $addFields overwrites the existing value of that field with the value of the specified expression.如果新字段的名称与现有字段名称（包括_id）相同，$addFields将用指定表达式的值覆盖该字段的现有值。

Behavior行为

  • $addFields appends new fields to existing documents. 将新字段附加到现有文档中。You can include one or more $addFields stages in an aggregation operation.您可以在聚合操作中包含一个或多个$addFields阶段。
  • $addFields accepts the embedding of objects where you can set a value to an aggregation expression or to an empty object. 接受对象的嵌入，您可以在其中为聚合表达式或空对象设置值。For example, the following nested objects are accepted:例如，接受以下嵌套对象：

    {$addFields: { a: { b: { } } } }

    To add a field or fields to embedded documents (including documents in arrays) use the dot notation. 要向嵌入文档（包括数组中的文档）添加一个或多个字段，请使用点表示法。See example.请参见示例。

  • To add an element to an existing array field with $addFields, use with $concatArrays. See example.要使用$addFields将元素添加到现有数组字段中，请与$concatArrays一起使用。请参见示例。

Examples实例

Using Two $addFields Stages使用两个$addFields阶段

A collection called scores contains the following documents:一个名为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: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向嵌入文档添加字段

Use dot notation to add new fields to embedded documents.使用点表示法将新字段添加到嵌入文档中。

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字段fuel_type添加到嵌入的文档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的集合包含以下文档：

{ _id: 1, dogs: 10, cats: 15 }

The following $addFields operation specifies the cats field.下面的$addFields操作指定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 }

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.在下面的示例中，item字段替换_id字段。

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

{ "_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字段替换成静态值。

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

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

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

Add Element to an Array将元素添加到数组

Create a sample scores collection with the following:创建一个包含以下内容的样例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 $concatArrays expression to add an element to an existing array field. 您可以将$addFields与$concatArrays表达式一起使用，将元素添加到现有的数组字段中。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，该数组的元素是与包含新分数[7]的另一个数组连接的当前homework数组。

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 }