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$addToSet (aggregation)

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Definition定义

Changed in version 5.0.5.0版更改。

$addToSet

$addToSet returns an array of all unique values that results from applying an expression to each document in a group.返回将表达式应用于组中的每个文档所产生的所有唯一值的数组。

The order of the elements in the returned array is unspecified.返回数组中元素的顺序未指定。

$addToSet is available in these stages:可在以下阶段使用：

Syntax语法

$addToSet syntax:语法：

{ $addToSet: <expression> }

For more information on expressions, see Expressions.有关表达式的详细信息，请参阅表达式。

Behavior行为

If the value of the expression is an array, $addToSet appends the whole array as a single element.如果表达式的值是一个数组，$addToSet会将整个数组附加为单个元素。

If the value of the expression is a document, MongoDB determines that the document is a duplicate if another document in the array matches the to-be-added document exactly. 如果表达式的值是一个文档，那么如果数组中的另一个文档与要添加的文档完全匹配，MongoDB就会确定该文档是重复的。Specifically, the existing document has the exact same fields and values in the exact same order.具体来说，现有文档具有完全相同的字段和值，其顺序完全相同。

Examples实例

Use in $group Stage在$group阶段中使用

Consider a sales collection with the following documents:考虑一个包含以下文档的sales集合：

{ "_id" : 1, "item" : "abc", "price" : 10, "quantity" : 2, "date" : ISODate("2014-01-01T08:00:00Z") }
{ "_id" : 2, "item" : "jkl", "price" : 20, "quantity" : 1, "date" : ISODate("2014-02-03T09:00:00Z") }
{ "_id" : 3, "item" : "xyz", "price" : 5, "quantity" : 5, "date" : ISODate("2014-02-03T09:05:00Z") }
{ "_id" : 4, "item" : "abc", "price" : 10, "quantity" : 10, "date" : ISODate("2014-02-15T08:00:00Z") }
{ "_id" : 5, "item" : "xyz", "price" : 5, "quantity" : 10, "date" : ISODate("2014-02-15T09:12:00Z") }

Grouping the documents by the day and the year of the date field, the following operation uses the $addToSet accumulator to compute the list of unique items sold for each group:根据date字段的日期和年份对文档进行分组，以下操作使用$addToSet累加器来计算每组销售的唯一项目列表：

db.sales.aggregate(
   [
     {
       $group:
         {
           _id: { day: { $dayOfYear: "$date"}, year: { $year: "$date" } },
           itemsSold: { $addToSet: "$item" }
         }
     }
   ]
)

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

{ "_id" : { "day" : 46, "year" : 2014 }, "itemsSold" : [ "xyz", "abc" ] }
{ "_id" : { "day" : 34, "year" : 2014 }, "itemsSold" : [ "xyz", "jkl" ] }
{ "_id" : { "day" : 1, "year" : 2014 }, "itemsSold" : [ "abc" ] }

Use in $setWindowFields Stage在$setWindowFields阶段中使用

New in version 5.0. 5.0版新增。

Create a cakeSales collection that contains cake sales in the states of California (CA) and Washington (WA):创建一个包含加利福尼亚州（CA）和华盛顿州（WA）蛋糕销售的cakeSales系列：

db.cakeSales.insertMany( [
   { _id: 0, type: "chocolate", orderDate: new Date("2020-05-18T14:10:30Z"),
     state: "CA", price: 13, quantity: 120 },
   { _id: 1, type: "chocolate", orderDate: new Date("2021-03-20T11:30:05Z"),
     state: "WA", price: 14, quantity: 140 },
   { _id: 2, type: "vanilla", orderDate: new Date("2021-01-11T06:31:15Z"),
     state: "CA", price: 12, quantity: 145 },
   { _id: 3, type: "vanilla", orderDate: new Date("2020-02-08T13:13:23Z"),
     state: "WA", price: 13, quantity: 104 },
   { _id: 4, type: "strawberry", orderDate: new Date("2019-05-18T16:09:01Z"),
     state: "CA", price: 41, quantity: 162 },
   { _id: 5, type: "strawberry", orderDate: new Date("2019-01-08T06:12:03Z"),
     state: "WA", price: 43, quantity: 134 }
] )

This example uses $addToSet in the $setWindowFields stage to output the unique cake type sales for each state:此示例使用$setWindowFields阶段中的$addToSet来输出每个state的唯一蛋糕type销售额：

db.cakeSales.aggregate( [
   {
      $setWindowFields: {
         partitionBy: "$state",
         sortBy: { orderDate: 1 },
         output: {
            cakeTypesForState: {
               $addToSet: "$type",
               window: {
                  documents: [ "unbounded", "current" ]
               }
            }
         }
      }
   }
] )

In the example:在示例中：

  • partitionBy: "$state" partitions the documents in the collection by state. 按state对集合中的文档进行分区。There are partitions for CA and WA.CA和WA有分区。
  • sortBy: { orderDate: 1 } sorts the documents in each partition by orderDate in ascending order (1), so the earliest orderDate is first.按orderDate按升序（1）对每个分区中的文档进行排序，因此最早的orderDate是第一个。
  • output adds each unique cake type to the cakeTypesForState array field using $addToSet that is run in a documents window.output使用在documents窗口中运行的$addToSet将每个唯一的蛋糕类型添加到cakeTypesForState数组字段中。

    The window contains documents between an unbounded lower limit and the current document. 该window包含介于unbounded下限和current文档之间的文档。This means $addToSet returns an array containing the unique cake type fields for the documents between the beginning of the partition and the current document.这意味着$addToSet返回一个数组，该数组包含分区开头和当前文档之间文档的唯一蛋糕type字段。

In this example output, the cake type array for CA and WA is shown in the cakeTypesForState field:在此示例输出中，CA和WA的cakeTypesForState字段中显示了cakeTypes数组：

{ "_id" : 4, "type" : "strawberry", "orderDate" : ISODate("2019-05-18T16:09:01Z"),
  "state" : "CA", "price" : 41, "quantity" : 162,
  "cakeTypesForState" : [ "strawberry" ] }
{ "_id" : 0, "type" : "chocolate", "orderDate" : ISODate("2020-05-18T14:10:30Z"),
  "state" : "CA", "price" : 13, "quantity" : 120,
  "cakeTypesForState" : [ "strawberry", "chocolate" ] }
{ "_id" : 2, "type" : "vanilla", "orderDate" : ISODate("2021-01-11T06:31:15Z"),
  "state" : "CA", "price" : 12, "quantity" : 145,
  "cakeTypesForState" : [ "strawberry", "vanilla", "chocolate" ] }
{ "_id" : 5, "type" : "strawberry", "orderDate" : ISODate("2019-01-08T06:12:03Z"),
  "state" : "WA", "price" : 43, "quantity" : 134,
  "cakeTypesForState" : [ "strawberry" ] }
{ "_id" : 3, "type" : "vanilla", "orderDate" : ISODate("2020-02-08T13:13:23Z"),
  "state" : "WA", "price" : 13, "quantity" : 104,
  "cakeTypesForState" : [ "vanilla", "strawberry" ] }
{ "_id" : 1, "type" : "chocolate", "orderDate" : ISODate("2021-03-20T11:30:05Z"),
  "state" : "WA", "price" : 14, "quantity" : 140,
  "cakeTypesForState" : [ "vanilla", "chocolate", "strawberry" ] }