$in（聚合）
定义
$in返回一个布尔值，指示指定的值是否在数组中。
{ $in: [ <expression>, <array expression> ] } 与
$in查询运算符不同，聚合$in运算符不支持通过 正则表达式 匹配。示例结果{ $in: [ 2, [ 1, 2, 3 ] ] }true{ $in: [ "abc", [ "xyz", "abc" ] ] }true{ $in: [ "xy", [ "xyz", "abc" ] ] }false{ $in: [ [ "a" ], [ "a" ] ] }false{ $in: [ [ "a" ], [ [ "a" ] ] ] }true{ $in: [ /^a/, [ "a" ] ] }false{ $in: [ /^a/, [ /^a/ ] ] }true
行为
$in 在以下任一情况下会出错：如果 $in 表达式未给出恰好两个参数，或者第二个参数的结果不是数组。
示例
名为 fruit 的集合包含以下文档
{ "_id" : 1, "location" : "24th Street", "in_stock" : [ "apples", "oranges", "bananas" ] } { "_id" : 2, "location" : "36th Street", "in_stock" : [ "bananas", "pears", "grapes" ] } { "_id" : 3, "location" : "82nd Street", "in_stock" : [ "cantaloupes", "watermelons", "apples" ] }
以下聚合操作检查每个文档中的 in_stock 数组，以确定字符串 bananas 是否存在。
db.fruit.aggregate([ { $project: { "store location" : "$location", "has bananas" : { $in: [ "bananas", "$in_stock" ] } } } ])
操作返回以下结果
{ "_id" : 1, "store location" : "24th Street", "has bananas" : true } { "_id" : 2, "store location" : "36th Street", "has bananas" : true } { "_id" : 3, "store location" : "82nd Street", "has bananas" : false }