MongoDB: Graphlookup nested documents only returns single document in aggregation

Question

I'm trying the MongoDB aggregation framework to work with nested documents but having trouble returning the expected output, specifically in the $graphLookup stage. In a non-nested schema, it correctly looks up all the documents as defined in the options and returns all. But in a nested one, it only returns one. I have tried $unwind and $replaceRoot as answered here but now it does not work. It would be more understandable through code so here are the samples.

Non-nested document (fileSystem does not count)

db={
  "fileSystem": [
    {
      "_id": "a",
      "label": "Root",
      "children": [
        "b",
      ],
    },
    {
      "_id": "b",
      "label": "Nested folder 1",
      "children": [
        "c",
        "d",
        "e"
      ],
      "parent": "a"
    },
    {
      "_id": "c",
      "label": "Nested File 1.1",
      "parent": "b"
    },
    {
      "_id": "d",
      "label": "Nested File 1.2",
      "parent": "b"
    },

  ]
}

// Aggregation Query

db.fileSystem.aggregate([
  {
    "$match": {
      "_id": "a"
    }
  },
  {
    "$graphLookup": {
      "from": "fileSystem",
      "startWith": "$children",
      "connectFromField": "children",
      "connectToField": "_id",
      "as": "nest",
      "depthField": "level",
      "maxDepth": 1
    }
  },
])

// correct and expected result

[
  {
    "_id": "a",
    "children": [
      "b"
    ],
    "label": "Root",
    "nest": [
      {
        "_id": "b",
        "children": [
          "c",
          "d",
          "e"
        ],
        "label": "Nested folder 1",
        "level": NumberLong(0),
        "parent": "a"
      },
      {
        "_id": "d",
        "label": "Nested File 1.2",
        "level": NumberLong(1),
        "parent": "b"
      },
      {
        "_id": "c",
        "label": "Nested File 1.1",
        "level": NumberLong(1),
        "parent": "b"
      }
    ]
  }
]

Nested document and query

db={
  "fileSystem": [
    {
      pp: [
        {
          "_id": "a",
          "label": "Root",
          "children": [
            "b",
          ],
        },
        // ... same as previous
      ]
    }
  ]
}

// Aggregation Query

db.fileSystem.aggregate([
  {
    "$unwind": "$pp"
  },
  {
    "$replaceRoot": {
      "newRoot": "$pp"
    }
  },
  {
    "$match": {
      "_id": "a"
    }
  },
  {
    "$graphLookup": {
      "from": "fileSystem",
      "startWith": "$pp.children",
      "connectFromField": "pp.children",
      "connectToField": "pp._id",
      "as": "nest",
      "depthField": "level",

    }
  }, 
])

// incorrect result

[
  {
    "_id": "a",
    "children": [
      "b"
    ],
    "label": "Root",
    "nest": []
  }
]

Expected: https://mongoplayground.net/p/A4yDGUHka58

Bugged: https://mongoplayground.net/p/ZlQyDBrYSZr

Answer 1

$graphLookup searches the collection given in from for matching documents. It uses each document in the pipeline as a starting point, but it does not search for, and will not return documents from the pipline.

In the sample data there is there is only 1 document, so the best you'll get in that case is for the next array to contain the original document.

Playground