Return $sample result as appended JSON key MongoDB

December 22, 2022

I perform a query to find one specific record by ID in MongoDB. It works fine, and returns me the following document:

{
   key1: "value1",
   key2: "value2"
}

Now I need to add to the returned JSON record the key "seeAlso".

This key will contain a sample of 3 other records in the same collection.

The final document should look like it:

{
   key1: "value1",
   key2: "value2",
   seeAlso: [
      {doc1},
      {doc2},
      {doc3},
   ]
}

The $sample I already know how to do. Gotta add the following to the pipeline:

  {
    '$sample': {
      'size': 10
    }
  }

What I don’t know yet is how to store the result of this sample in the "seeAlso" key. Such key needs to be created at this point, it doesn’t yet exist.

>Solution :

You may just need to do the $sample in a subpipeline. To leave out the original record, add a $match at the start of the subpipeline to leave out by _id

db.collection.aggregate([
  {
    "$match": {
      key1: "value1"
    }
  },
  {
    "$lookup": {
      "from": "collection",
      "let": {
        id: "$_id"
      },
      "pipeline": [
        {
          $match: {
            $expr: {
              $ne: [
                "$_id",
                "$$id"
              ]
            }
          }
        },
        {
          "$sample": {
            "size": 3
          }
        }
      ],
      "as": "seeAlso"
    }
  }
])

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