Mongodb (aggregate) filter nested array based on array in the document

Question

I have a mongo pipeline. Here is the output of one of the steps.

    {
  "_id": "6249b7a8338c31b803a1e56b",
  "sentences": [
    66,
    61,
    98,
    44
  ],
  "documents": [
    {
      "sentences": [
        {
          "uuid": 66,
          "text": "cbElZuplrxPQicnBHvKQutEhZ",
          "index": 58,
          "cluster_id": "PyYvsHQfnypoowYsswiAJLlFP"
        },
        {
          "uuid": 61,
          "text": "cbElZuplrxPQicnBHvKQutEhZ",
          "index": 58,
          "cluster_id": "PyYvsHQfnypoowYsswiAJLlFP"
        }
      ]
    }
  ]
}

In one of the steps, I need to filter the array $documents.sentences based on whether the $documents.sentences.uuid is in $sentences. My pipeline $project stage is defined like this:

{
  "sentences": 1,
  "documents.sentences": {
    $filter: { 
      "input": "$documents.sentences", 
      "as": "s", 
      "cond": {
        $in: ["$$s.uuid", "$sentences"],
      }}
    
  }
}

However, this results in a completely empty $documents My question is, what is the best way to filter the documents.sentences array based on the condition, if documents.sentences.uuid is in sentences?

thank you for your time

Adrien D · Accepted Answer · 2022-04-04 06:49:58Z

1

As the sequences array is in an array of documents, you need to include an intermediary $map operation:

db.collection.aggregate([
  {
    "$project": {
      "sentences": 1,
      "documents": {
        $map: {
          "input": "$documents",
          "as": "d",
          "in": {
            "sequences": {
              $filter: {
                "input": "$$d.sentences",
                "as": "s",
                "cond": {
                  $in: [
                    "$$s.uuid",
                    "$sentences"
                  ],
                  
                }
              }
            }
          }
        }
      }
    }
  }
])

answered Apr 4, 2022 at 6:49

Adrien D

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1 Comment

bell_pepper Over a year ago

Thank you @Adrien D. That is the correct answer, when I can accept, I will. Thank you so much for your help.

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