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$setEquals（表达式操作符）

定义

$setEquals

比较两个或多个数组，如果它们具有相同的不同元素，则返回 true，否则返回 false。

$setEquals 通过以下语法实现：

{ $setEquals: [ <expression1>, <expression2>, ... ] }

参数可以是任何有效的表达式，只要它们都解析为一个数组即可。有关表达式的更多信息，请参阅表达式。

行为

$setEquals performs set operation on arrays, treating arrays as sets. If an array contains duplicate entries, $setEquals ignores the duplicate entries. $setEquals ignores the order of the elements.

If a set contains a nested array element, $setEquals does not descend into the nested array but evaluates the array at top-level.

例子

结果

注意

{ $setEquals: [
  [ "a", "c" ],
  [ "a", "b" ]
] }

false

这两个数组不包含相同的元素。

{ $setEquals: [
  [ "a", "c" ],
  [ "a", "b", "c" ]
] }

false

这两个数组不包含相同的元素。

{ $setEquals: [
  [ "a", "b", "a" ],
  [ "b", "a" ]
] }

true

两个数组都包含相同的唯一元素。当视为集合时，第一个大量为 [ "a", "b" ]，因为重复项将被忽略。

{ $setEquals: [
  [ "a", "b" ],
  [ [ "a", "b" ] ]
] }

false

数组中的元素不相等。第一个大量包含字符串 "a" 和 "b"，第二个数组包含大量[ "a", "b" ]。

{ $setEquals: [
  [ ],
  [ "a", "b" ]
] }

false

数组中的元素不相等。第一个大量为空，第二个大量有两个字符串元素。

{ $setEquals: [
  [ "a", "a" ],
  [ "a", "b" ]
] }

false

这些数组不包含相同的唯一元素。第一个大量仅包含 "a"，而第二个大量包含 "a" 和 "b"。

例子

请考虑包含以下文档的 bakeryOrders 集合：

db.bakeryOrders.insertMany( [
   { _id: 0, cakes: ["chocolate", "vanilla"], cupcakes: ["chocolate", "vanilla"] },
   { _id: 1, cakes: ["chocolate", "vanilla"], cupcakes: ["vanilla", "chocolate"] },
   { _id: 2, cakes: ["chocolate", "chocolate"], cupcakes: ["chocolate"] },
   { _id: 3, cakes: ["vanilla"], cupcakes: ["chocolate"] },
   { _id: 4, cakes: ["vanilla"], cupcakes: [] }
] )

The following operation uses the $setEquals operator to determine if the cakes array and the cupcakes array in each order contain the same flavors:

db.bakeryOrders.aggregate(
   [
      {
         $project: {
            _id: 0,
            cakes: 1,
            cupcakes: 1,
            sameFlavors: { $setEquals: [ "$cakes", "$cupcakes" ] }
         }
      }
   ] )

注意

$project

$project 阶段指定输出文档中包含哪些字段。在此示例中，$project 阶段：

从输出中排除 _id 字段。
在输出中包含 cakes 和 cupcakes 字段。
在名为 sameFlavors 的新字段中输出 $setEquals 操作符的结果。

操作返回以下结果：

{
 cakes: [ "chocolate", "vanilla" ],
 cupcakes: [ "chocolate", "vanilla" ],
 sameFlavors: true
},
{
 cakes: [ "chocolate", "vanilla" ],
 cupcakes: [ "vanilla", "chocolate" ],
 sameFlavors: true
},
{
 cakes: [ "chocolate", "chocolate" ],
 cupcakes: [ "chocolate" ],
 sameFlavors: true
},
{
 cakes: [ "vanilla" ],
 cupcakes: [ "chocolate" ],
 sameFlavors: false
},
{
   cakes: [ "vanilla" ],
   cupcakes: [],
   sameFlavors: false
}