Welcome to OGeek Q&A Community for programmer and developer-Open, Learning and Share
Welcome To Ask or Share your Answers For Others

Categories

0 votes
1.3k views
in Technique[技术] by (71.8m points)

mongodb - $project: Is it possible to access a property of an expression result in just one single stage?

Is there any way to carry out the following operation in just one $project stage?

db.getCollection('users').aggregate([
    {
        $project : {
            firstEmail : { $arrayElemAt : ["$emails", 0] }
        }
    },
    {
        $project : {
            domain : "$firstEmail.domain"
        }
    }
])
See Question&Answers more detail:os

与恶龙缠斗过久,自身亦成为恶龙;凝视深渊过久,深渊将回以凝视…
Welcome To Ask or Share your Answers For Others

1 Reply

0 votes
by (71.8m points)

You need $let operator:

db.getCollection('users').aggregate([
    {
        $project : {
            domain : {
                $let: {
                    vars: { firstEmail: { $arrayElemAt : ["$emails", 0] } },
                    in: "$$firstEmail.domain"
                }
            }
        }
    }
])

Mongo Playground


与恶龙缠斗过久,自身亦成为恶龙;凝视深渊过久,深渊将回以凝视…
OGeek|极客中国-欢迎来到极客的世界,一个免费开放的程序员编程交流平台!开放,进步,分享!让技术改变生活,让极客改变未来! Welcome to OGeek Q&A Community for programmer and developer-Open, Learning and Share
Click Here to Ask a Question

...