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
1.6k views
in Technique[技术] by (71.8m points)

mongodb - Get first element in array and return using Aggregate?

How can I get and return the first element in an array using a Mongo aggregation?

I tried using this code:

db.my_collection.aggregate([
    { $project: {
        resp : { my_field: { $slice: 1 } }
    }}
])

but I get the following error:

uncaught exception: aggregate failed: {
    "errmsg" : "exception: invalid operator '$slice'",
    "code" : 15999,
    "ok" : 0
}

Note that 'my_field' is an array of 4 elements, and I only need to return the first element.

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)

Since 3.2, we can use $arrayElemAt to get the first element in an array

db.my_collection.aggregate([
    { $project: {
        resp : { $arrayElemAt: ['$my_field',0] }
    }}
])

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

...