MongoDB projection has $slice how to display only this field

A simple projection

A little use of mongodb know that the projection is very simple, directly

db.student.find({_id:ObjectId(‘5a5085aed8f10c1a6cc0395b‘)},{comments: 1})

Add a $slice projection

However, when I want to give comments ( $slice ) How to do it?

The wrong approach

The following are the wrong practices

db.student.find({_id:ObjectId(‘5a5085aed8f10c1a6cc0395b‘)},{comments: 1, comments:{$slice:[0,1]}})

In this case, only the pagination, and then the fields Show All .

reason :
Object, the same name field, which overrides the former. So the only thing that {comments: 1, comments: {$slice:[0,1]}} actually comes into effect is comments:{$slice:[0,1]} .
Similarly, if the two swap positions become {comments: {$slice:[0,1]}, comments: 1} , then the actual effect is comments: 1 that there is no paging .

The correct wording

$elemMatch, can do

db.student.find({_id:ObjectId(‘5a5085aed8f10c1a6cc0395b‘)}, {comments:{$slice:[0,1]},  $elemMatch:1})

For $elemMatch , this usage is not described in the official website documentation. So I don't know if this is astray , haha.

There are other more elegant wording to tell

How to use the spring

In spring, query is assembled as follows. Use the normal include to write.

Query query = new Query();query.addCriteria(Criteria.where("_id").is(new ObjectId("5a5085aed8f10c1a6cc0395b")));query.fields().slice("comments", 0, 1);query.fields().include("$elemMatch");

The wrong way of spelling

query.fields().slice("comments", 0, 1).elemMatch("comments", new Criteria());

So the spell will turn out

db.student.find({_id:ObjectId(‘5a5085aed8f10c1a6cc0395b‘)}, {comments:{$slice:[0,1]},  $elemMatch: {}})

And this statement will be an error if directly executed in MongoDB.

MongoDB projection has $slice how to display only this field