The output of the db.name.aggregate() function gives output:

[{}, {"abc": "zyx"}, {}, "opk": "tyr"]

Actual output desired :

[{"abc": "zyx"}, "opk": "tyr"]

CodePudding user response：

Firstly, your output is not a valid array. It should be like this:

[{}, {"abc": "zyx"}, {}, {"opk": "tyr"}]

Now, to obtain your desired output, you can add the following $match stage, to your pipeline:

db.collection.aggregate([
  {
    "$match": {
      $expr: {
        "$gt": [
          {
            "$size": {
              "$objectToArray": "$$ROOT"
            }
          },
          0
        ]
      }
    }
  }
])

Here, we are converting the document to an array using $objectToArray, and then we check whether the size of that array is greater than 0. Only those documents are kept in the output.

Playground link.

CodePudding user response：

What if you data looks like this.

[
  {
    "arr": [
      {},
      {
        "abc": "zyx"
      },
      {},
      {
        "opk": "tyr"
      }
    ]
  }
]

The aggregation be like this to remove empty objects

db.collection.aggregate([
  {
    "$unwind": {
      "path": "$arr",
      
    }
  },
  {
    "$match": {
      arr: {
        "$ne": {}
      }
    }
  },
  {
    "$group": {
      _id: "$_id",
      arr: {
        $push: "$arr"
      }
    }
  }
])

Outputs

[
  {
    "_id": ObjectId("5a934e000102030405000000"),
    "arr": [
      {
        "abc": "zyx"
      },
      {
        "opk": "tyr"
      }
    ]
  }
]

Demo@mongoplayground

https://mongoplayground.net/p/BjGxzlrlj6s