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$allElementsTrue（表达式操作符）

定义

$allElementsTrue

将数组作为集合进行计算，如果数组中true 没有元素为，则返回false 。否则，返回false 。空数组返回true 。

$allElementsTrue 通过以下语法实现：

{ $allElementsTrue: [ <expression> ] }

<expression>本身必须解析为一个数组，与表示参数列表的外部数组分开。有关表达式的更多信息，请参阅表达式。

行为

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

In addition to the false boolean value, $allElementsTrue evaluates as false the following: null, 0, and undefined values. The $allElementsTrue evaluates all other values as true, including non-zero numeric values and arrays.

例子	结果
`{ $allElementsTrue: [ [ true, 1, "someString" ] ] }`	`true`
`{ $allElementsTrue: [ [ [ false ] ] ] }`	`true`
`{ $allElementsTrue: [ [ ] ] }`	`true`
`{ $allElementsTrue: [ [ null, false, 0 ] ] }`	`false`

例子

使用以下文档创建名为survey的示例collection：

db.survey.insertMany([
   { "_id" : 1, "responses" : [ true ] },
   { "_id" : 2, "responses" : [ true, false ] },
   { "_id" : 3, "responses" : [ ] },
   { "_id" : 4, "responses" : [ 1, true, "seven" ] },
   { "_id" : 5, "responses" : [ 0 ] },
   { "_id" : 6, "responses" : [ [ ] ] },
   { "_id" : 7, "responses" : [ [ 0 ] ] },
   { "_id" : 8, "responses" : [ [ false ] ] },
   { "_id" : 9, "responses" : [ null ] },
   { "_id" : 10, "responses" : [ undefined ] }
])

The following operation uses the $allElementsTrue operator to determine if the responses array only contains values that evaluate to true:

db.survey.aggregate(
   [
     { $project: { responses: 1, isAllTrue: { $allElementsTrue: [ "$responses" ] }, _id: 0 } }
   ]
)

操作返回以下结果：

{ "responses" : [ true ], "isAllTrue" : true }
{ "responses" : [ true, false ], "isAllTrue" : false }
{ "responses" : [ ], "isAllTrue" : true }
{ "responses" : [ 1, true, "seven" ], "isAllTrue" : true }
{ "responses" : [ 0 ], "isAllTrue" : false }
{ "responses" : [ [ ] ], "isAllTrue" : true }
{ "responses" : [ [ 0 ] ], "isAllTrue" : true }
{ "responses" : [ [ false ] ], "isAllTrue" : true }
{ "responses" : [ null ], "isAllTrue" : false }
{ "responses" : [ undefined ], "isAllTrue" : false }