On this page本页内容
$set
New in version 4.2.在版本4.2中新增。
Adds new fields to documents. 向文档中添加新字段$设置包含输入文档和新添加字段中所有现有字段的输出文档。$set outputs documents that contain all existing fields from the input documents and newly added fields.
The $set stage is an alias for $addFields.$set阶段是$addFields的别名。
Both stages are equivalent to a 这两个阶段都相当于$project stage that explicitly specifies all existing fields in the input documents and adds the new fields.$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。
$set has the following form:具有以下形式：
{ $set: { <newField>: <expression>, ... } }
Specify the name of each field to add and set its value to an aggregation expression. 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。For more information on expressions, see Expressions.有关表达式的详细信息，请参阅表达式。
If the name of the new field is the same as an existing field name (including 如果新字段的名称与现有字段名称（包括_id), $set overwrites the existing value of that field with the value of the specified expression._id）相同，$set将用指定表达式的值覆盖该字段的现有值。
$set appends new fields to existing documents. 将新字段附加到现有文档。You can include one or more 您可以在聚合操作中包括一个或多个$set stages in an aggregation operation.$set阶段。
To add field or fields to embedded documents (including documents in arrays) use the dot notation. 要向嵌入文档（包括数组中的文档）添加一个或多个字段，请使用点符号。See example.请参阅示例。
To add an element to an existing array field with 要使用$set, use with $concatArrays. $set将元素添加到现有数组字段中，请与$concatArrays一起使用。See example.参见示例。
$set Stages$set阶段Create a sample 创建具有以下内容的样本scores collection with the following: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 }
])
The following operation uses two 以下操作使用两个$set stages to include three new fields in the output documents:$set阶段在输出文档中包括三个新字段：
db.scores.aggregate( [
{
$set: {
totalHomework: { $sum: "$homework" },
totalQuiz: { $sum: "$quiz" }
}
},
{
$set: {
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
}
Use dot notation to add new fields to embedded documents.使用点符号将新字段添加到嵌入文档中。
Create a sample collection 创建具有以下内容的样本集合vehicles with the following:vehicles：
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 以下聚合操作将新字段fuel_type to the embedded document specs.fuel_type添加到嵌入式文档specs中。
db.vehicles.aggregate( [
{ $set: { "specs.fuel_type": "unleaded" } }
] )
The operation returns the following results:该操作返回以下结果：
{ _id: 1, type: "car", specs: { doors: 4, wheels: 4, fuel_type: "unleaded" } }
{ _id: 2, type: "motorcycle", specs: { doors: 0, wheels: 2, fuel_type: "unleaded" } }
{ _id: 3, type: "jet ski", specs: { fuel_type: "unleaded" } }
Specifying an existing field name in a 在$set operation causes the original field to be replaced.$set操作中指定现有字段名会导致替换原始字段。
Create a sample collection called 创建一个名为animals with the following:animals的样本集合，包括以下内容：
db.animals.insertOne( { _id: 1, dogs: 10, cats: 15 } )
The following 以下$set operation overrides the cats field:$set操作将覆盖cats字段：
db.animals.aggregate( [
{ $set: { "cats": 20 } }
] )
The operation returns the following document:操作将返回以下文档：
{ _id: 1, dogs: 10, cats: 20 }
It is possible to replace one field with another. 可以用另一个字段替换一个字段。In the following example the 在下面的示例中，item field substitutes for the _id field.item字段替换_id字段。
Create a sample collection called 创建一个名为fruits contains the following documents:fruits的样本集合，其中包含以下文档：
db.fruits.insertMany([
{ "_id" : 1, "item" : "tangerine", "type" : "citrus" },
{ "_id" : 2, "item" : "lemon", "type" : "citrus" },
{ "_id" : 3, "item" : "grapefruit", "type" : "citrus" }
])
The following aggregration operation uses 下面的聚合操作使用$set to replace the _id field of each document with the value of the item field, and replaces the item field with a string "fruit".$set将每个文档的_id字段替换为item字段的值，并将item字段替换为字符串"fruit"。
db.fruits.aggregate( [
{ $set: { _id : "$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" }
Create a sample 创建具有以下内容的样本scores collection with the following: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 可以将$set with a $concatArrays expression to add an element to an existing array field. $set与$concatArrays表达式一起使用，将元素添加到现有数组字段中。For example, the following operation uses 例如，以下操作使用$set to replace the homework field with a new array whose elements are the current homework array concatenated with another array containing a new score [ 7 ].$set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 } },
{ $set: { 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 }