Using $project to Change Data Structure Returned in MongoDB

Question

I have a function in my MongoDB/Node backend that returns a list of departments.

The documents in the collection look like this:

[
    {
      _id: 111,
      department: "abc"
    },
    {
      _id: 222,
      department: "def"
    },
    {
      _id: 333,
      department: "ghi"
    }
]

This is working with aggregation looks like this:

  $group: {
    _id: null,
    data: { $addToSet: "$department" }
  }

However, what this produces is not ideal. The output looks like this:

{
    "count": 1,
    "data": [
        {
            "_id": null,
            "data": [
                "abc",
                "def",
                "ghi"
            ]
        }
    ]
}

What I'd like to do is return data where there isn't a nested array structure with "data" inside "data". I'd like this output:

{
  "count": 1,
  "data": [
    "abc",
    "def",
    "ghi",
  ]
}

Is there a way I can do this with a projection stage?

I tried this:

  {
    $group: {
      _id: null,
      data: { $addToSet: "$department" }
    }
  },
  {
    $project: {
      data: 0,
      _id: 0
    }
  }

But end up with the same data structure returned. Is there a way I can do this with $project?

UPDATE:

After a suggestion I tried this:

db.staffmembers.aggregate([
      {
        $group: {
          _id: null,
          data: { $addToSet: "$department" }
        }
      },
      {
        $project: {
          data: {
            $reduce: {
              input: "$data.data",
              initialValue: [],
              in: {
                $concatArrays: ["$$this", "$$value"]
              }
            }
          }
        }
      }
    ]);

... but this outputs an empty array for data:

{ 
    "_id" : null, 
    "data" : [

    ]
}

post some sample data

Senthur Deva
– Senthur Deva

2018-10-04 13:57:34 +00:00
Commented Oct 4, 2018 at 13:57 — Senthur Deva
– Senthur Deva, Commented Oct 4, 2018 at 13:57

dnickless · Accepted Answer · 2018-10-04 21:30:05Z

1

This is what you're after:

db.collection.aggregate({
    $project: {
        data: { $arrayElemAt: [ "$data.data", 0 ] },
        _id: 0
    }
})

Mind you, I have a feeling there's something funky about your pipeline and/or data setup. It would probably be beneficial if you could share the input data and the entire pipeline.

answered Oct 4, 2018 at 21:30

dnickless

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Comments

Rakshith Murukannappa · Accepted Answer · 2018-10-04 15:04:11Z

0

You can use the $project stage in your pipeline to cherry pick what you require from the previous stage of the aggregation

db.staffmembers.aggregate([
    {
        $group:{
            _id:null,
            data:{$addToSet:"$department"}
        }
    },
    {
        $project:{
            data:1,
            _id:0
        }
    }
])

answered Oct 4, 2018 at 15:04

Rakshith Murukannappa

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