MongoDB query for capped sub-collection in an array

In MongoDB, you cannot use capped for sub-collection. However, use capped on the overall document. To display a specific number of values from an array, prefer $slice.

Let us create a collection with documents −

> db.demo319.insertOne({"Scores":[100,345,980,890]}); {    "acknowledged" : true,    "insertedId" : ObjectId("5e50ecf6f8647eb59e562064") } > db.demo319.insertOne({"Scores":[903,10004,84575,844]}); {    "acknowledged" : true,    "insertedId" : ObjectId("5e50ed01f8647eb59e562065") }

Display all documents from a collection with the help of find() method −

> db.demo319.find().pretty();

This will produce the following output −

{    "_id" : ObjectId("5e50ecf6f8647eb59e562064"),    "Scores" : [       100,       345,       980,       890    ] } {    "_id" : ObjectId("5e50ed01f8647eb59e562065"),    "Scores" : [       903,       10004,       84575,       844    ] }

Following is the query for capped sub-collection in an array −

> db.demo319.aggregate([ ... { $project: {TwoScores: { $slice: [ "$Scores", 2 ] } } } ... ])

This will produce the following output −

{ "_id" : ObjectId("5e50ecf6f8647eb59e562064"), "TwoScores" : [ 100, 345 ] } { "_id" : ObjectId("5e50ed01f8647eb59e562065"), "TwoScores" : [ 903, 10004 ] }