$addToSet返回一个无重复元素的数组，元素值是对每个分组文档执行表达式的结果。数组元素顺序未指定。

$addToSet可以用于下列聚合阶段：

  • $bucket
  • $bucketAuto
  • $group
  • $setWindowFeilds

语法

{ $addToSet: }

用法

  • 如果表达式的值是个数组，$addToSet会把整个数组当成一个元素添加到返回的数组。
  • 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 就会判定该文档为重复文档。具体来说，现有文档具有完全相同的字段和值，且顺序完全相同。

举例

在$group阶段中使用

有一个sales集合包含以下文档：

{ "_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") }

根据date字段按照日和年对文档进行分组，下面的操作使用$addToSet将分组内不重复的销售物品作为列表返回：

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

操作返回下面的结果：

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

在$setWindowFields中使用

创建一个cakeSales集合，包含蛋糕在California (CA)和Washington (WA)的销售记录：

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 }] )

下面在$setWindowFields集合中使用$addToSet，输出在每个州销售的不重复的蛋糕类型:

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

本例中：

  • partitionBy: "$state"根据state对集合文档进行分区，分为CA和WA两个分区。
  • sortBy: { orderDate: 1 }将每个分区中的文件按orderDate升序排序（1），最早的orderDate排在前面。
  • output使用运行在文档窗口的$addToSet把每个唯一的蛋糕类型type添加到cakeTypesForState数组字段。
    该窗口包含介于无限制下限unbounded和当前current文档之间的文档，这意味着$addToSet将返回一个数组，该数组包含分区开头和当前文档之间文档的唯一蛋糕类型type。

本例在CA和WA的cakeTypesForState字段输出蛋糕类型type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" ] }