Flatten a map with documents having an embedded array in mongodb

Question

Through an aggregation pipeline, I get a list of objects, with a conditional checkin list:

/* 1 */
{
    "_id" : ObjectId("600ebcb7f1128cd4517f2336"),
    "deskId" : "deskA",
    "checkin" : []
}

/* 2 */
{
    "_id" : ObjectId("600ebcb7f1128cd4517f2338"),
    "deskId" : "deskB",
    "checkin" : [ 
        {
            "_id" : ObjectId("602a7a4483e6fe1a7f5fe909"),
            "date" : ISODate("2021-02-15T13:42:28.712Z"),
            "phone" : "nico",
        }
    ]
}

Which I want to turn into

/* 1 */
{
    "_id" : ObjectId("600ebcb7f1128cd4517f2336"),
    "deskId" : "deskA",
    "phone" : ""
}

/* 2 */
{
    "_id" : ObjectId("600ebcb7f1128cd4517f2338"),
    "deskId" : "deskB",
    "phone" : "nico"
}

As in:

empty string when the array is empty
phone field when checkin.phone is present
At this stage of the pipeline, the checkin field of type array has zero or one element

I tried $unwind, but because the checkin field can be a list with zero element, top element are being skipped.

You can try using the preserveNullAndEmptyArrays option for $unwind. — prasad_
– prasad_, Commented Feb 15, 2021 at 14:21

Dheemanth Bhat · Accepted Answer · 2021-02-16 03:01:17Z

If you are sure that checkin array will always contain only zero/one value then below solution works (no need of $unwind operator):

db.users.aggregate([
    {
        $addFields: {
            "phone": {
                $ifNull: [{ $arrayElemAt: ["$checkin.phone", 0] }, ""]
            }
        }
    }
])

Output:

[
  {
    "_id": ObjectId("600ebcb7f1128cd4517f2336"),
    "deskId": "deskA",
    "phone": "",
    "checkin": []
  },
  {
    "_id": ObjectId("600ebcb7f1128cd4517f2338"),
    "deskId": "deskB",
    "phone": "nico",
    "checkin": [
      {
        "_id": ObjectId("602a7a4483e6fe1a7f5fe909"),
        "date": ISODate("2021-02-15T19:12:28.712+05:30"),
        "phone": "nico"
      }
    ]
  }
]

otherwise use below query:

db.users.aggregate([
    {
        $unwind: {
            path: "$checkin",
            preserveNullAndEmptyArrays: true
        }
    },
    {
        $group: {
            _id: "$_id",
            deskId: { $first: "$deskId" },
            phone: { $push: "$checkin.phone" }
        }
    }
])

Output:

[
  {
    "_id": ObjectId("600ebcb7f1128cd4517f2336"),
    "deskId": "deskA",
    "phone": []
  },
  {
    "_id": ObjectId("600ebcb7f1128cd4517f2338"),
    "deskId": "deskB",
    "phone": [
      "nico",
      "pico"
    ]
  }
]

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