如何在 Spring Data MongoDB 中使用 $arrayElemAt 运算符

本文介绍了如何在 Spring Data MongoDB 中使用 $arrayElemAt 运算符的处理方法，对大家解决问题具有一定的参考价值，需要的朋友们下面随着小编来一起学习吧！ 问题描述

$arrayElemAt MongoDB 3.2 版中的新内容.

$arrayElemAt new in MongoDB version 3.2.

db.users.aggregate([ { $project: { name: 1, first: { $arrayElemAt: [ "$favorites", 0 ] }, last: { $arrayElemAt: [ "$favorites", -1 ] } } }

])

推荐答案

DATAMONGO-1536 即将为 $arrayElemAt 和其他缺少的聚合运算符到 Spring Data MongoDB/Release-Train-Ingalls" rel="nofollow noreferrer">Ingalls 发布.同时你可以提供你自己的AggregationExpression来创建任何需要的操作.

DATAMONGO-1536 is about to add $arrayElemAt and other missing aggregation operators to Spring Data MongoDB for the Ingalls release. Meanwhile you can provide your own AggregationExpression to create whatever operation is needed.

对于上面的类似以下内容的技巧:

For the above something like the following does the trick:

project("name") // .and(context -> new BasicDBObject("$arrayElemAt", asList("$favorites", 0))).as("first") .and(context -> new BasicDBObject("$arrayElemAt", asList("$favorites", -1))).as("last");