Mongodb aggregation N random item from subdocument nested array-CodePudding

Good day all,

I'm trying to return some n random item from a nested array in an aggregation pipeline, however, I cannot seem to figure it out. all documentation from mongodb about random is at the document level, $sample which will return a number of documents based on the sample size defined. I want to return an n random number from an array within one document and then in a subsequent stage perform a $lookup stage. example below.

this is the document

{       
    _id: "61005f388308a717883ad3d2",
    ...................
    ...................
    "productLine": {
            "product": [
                    "5fefa2556308ab102854baf7",
                    "5fefa2556308ab102854baf8",
                    "5fefa2556308ab102854baf9",
                    "5fefa2556308ab102854bafc",
                    "5fefa2556308ab102854bag7",
                    "5fefa2556308ab102854bah8",
                    "5fefa2556308ab102854bac5",
                    "5fefa2556308ab102854babc"
            ],
            "created_count": 4,
            "purchased": [],
            "purchased_count": 0,
    },
    "following": {
            "users": [],
            "count": 0
    },
    "followers": {
            "users": [],
            "count": 0
    },
    "product_picture": "xxxxxxxxx",
    ....................
    ....................
    ....................
    
    "__v": 0
}

so in this document, I have 4 product id in the productLine, I want to be able to create a stage to select at random 2 of them so that in the following stage I can do the $lookup on these IDs. so basically after the stage, the document would look something like this.

{       
    _id: "61005f388308a717883ad3d2",
    ...................
    ...................
    "productLine": {
            "product": [
                    "5fefa2556308ab102854baf8", // this would be chosen at random
                    "5fefa2556308ab102854baf9", // this would be chosen at random
            ],
            "created_count": 4,
            "purchased": [],
            "purchased_count": 0,
    },
    "following": {
            "users": [],
            "count": 0
    },
    "followers": {
            "users": [],
            "count": 0
    },
    "product_picture": "xxxxxxxxx",
    ....................
    ....................
    ....................
    
    "__v": 0
}

the idea is when a user selects the product line, it will return two random product from that product line.

any help would be appreciated!!!

CodePudding user response：

You can perform the $sample in a subpipeline of $lookup.

{
    "$lookup": {
      "from": "product",
      "pipeline": [
        {
          "$project": {
            _id: 1
          }
        },
        {
          "$sample": {
            "size": 4
          }
        }
      ],
      "as": "productLookup"
    }
}

Here is the Mongo playground for your reference.