MongoDB mongoose $lookup not in array.

Question

I use the following query:

Collection
    .aggregate([
    {
        $project : { follow_count: {$size: { "$ifNull": [ "$follow_users", [] ] } } }
    },
    { $lookup: {from: 'models', localField: '_id', foreignField: '_id', as: 'model'}},
])

to get this kind of JSON:

[
  {
    "_id": "1234565434567",
    "follow_count": 3,
    "model": [
      {
        "_id": "1234565434567",
        "make": "Make1",
        "name": "Model1",
        "price": 15200,
      }
    ]
  },
  {
    "_id": "123456789",
    "follow_count": 2,
    "model": [
      {
        "_id": "123456789",
        "make": "Make2",
        "name": "Model2",
        "price": 12000,
      }
    ]
  }
]

Is there a way to don't push the $lookup result inside an array to have a JSON like that? I prefer to don't use loops after the query, so I am looking for an optimized way.

[
  {
    "_id": "1234565434567",
    "follow_count": 3,
    "make": "Make1",
    "name": "Model1",
    "price": 15200,
  },
  {
    "_id": "123456789",
    "follow_count": 2,
    "make": "Make2",
    "name": "Model2",
    "price": 12000,
  }
]

Try using $unwind or $project

Clement Amarnath
– Clement Amarnath

2018-03-08 10:49:30 +00:00
Commented Mar 8, 2018 at 10:49 — Clement Amarnath
– Clement Amarnath, Commented Mar 8, 2018 at 10:49

s7vr · Accepted Answer · 2018-03-08 13:13:31Z

1

You can use $mergeObject operator in 3.6 version.

$mergeObject to merge the joined collection fields with other fields followed by $replaceRoot to promote the combined doc to top level.

$project with exclusion to drop the model field.

Add the following stages after $lookup

 [
  {
    "$replaceRoot": {
      "newRoot": {
        "$mergeObjects": [
          {
            "$arrayElemAt": [
              "$model",
              0
            ]
          },
          "$$ROOT"
        ]
      }
    }
  },
  {
    "$project": {
      "model": 0
    }
  }
]

answered Mar 8, 2018 at 13:13

s7vr

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