获取MongoDB数组字段中给定元素的索引 [英] Get index of given element in array field in MongoDB

问题描述

想想这个 MongoDB 文档:

Think of this MongoDB document:

{_id:123, "food":[ "apple", "banana", "mango" ]}

问题:如何获取mango在食物中的位置?

Question: How to get the position of mango in food?

上面的查询应该返回2，不要返回整个文档.

The query should return 2 in above, and don't return the whole document.

请显示工作查询.

推荐答案

从 MongoDB 3.4 版开始，我们可以使用 $indexOfArray 运算符，用于返回可以在数组中找到给定元素的索引.

Starting from MongoDB version 3.4 we can use the $indexOfArray operator to return the index at which a given element can be found in the array.

$indexOfArray 接受三个参数.第一个是以$ 符号为前缀的数组字段的名称.

$indexOfArray takes three arguments. The first is the name of the array field prefixed with $ sign.

第二个是元素，第三个可选是开始搜索的索引.$indexOfArray 如果未指定开始搜索的索引，则返回找到该元素的第一个索引.

The second is the element and the third optional is the index to start the search at. $indexOfArray returns the first index at which the element is found if the index to start the search at is not specified.

演示:

> db.collection.insertOne( { "_id" : 123, "food": [ "apple", "mango", "banana", "mango" ] } )
{ "acknowledged" : true, "insertedId" : 123 }
> db.collection.aggregate( [ { "$project": { "matchedIndex": { "$indexOfArray": [ "$food", "mango" ] } } } ] )
{ "_id" : 123, "matchedIndex" : 1 }
> db.collection.aggregate( [ { "$project": { "matchedIndex": { "$indexOfArray": [ "$food", "mango", 2 ] } } } ] )
{ "_id" : 123, "matchedIndex" : 3 }
> db.collection.aggregate( [ { "$project": { "matchedIndex": { "$indexOfArray": [ "$food", "apricot" ] } } } ]  )
{ "_id" : 123, "matchedIndex" : -1 }

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