MongoDB Aggregation: Extract values from embedded Array And create single array on the root level

Question

Shape of the document(s) in the collection

[
  {
    "predictions": [
      "611b85b9b2b924245526c125"
    ],
    "_id": "611e40b6dd381f18cf63d15f",
   
    "__v": 0
  },
  {
    "predictions": [
      "611b85b9b2b924245526c125"
    ],
    "_id": "611e40b6dd381f18cf63d160",
    
    "__v": 0
  }
]

My Aggregation pipeline

        ActiveFixture.aggregate([
                {$match:{fixture:{$in:fixtureIds}}},
                {$project:{"predictions":1, _id:0}},
                {$unwind:"$predictions"},
                {$group:{_id:'blah', "Predictions":{"$push":"$predictions"}}}
               
               
                
        ]).exec()

Current output:

[
  {
    "_id": "blah",
    "Predictions": [
      "611b85b9b2b924245526c125",
      "611b85b9b2b924245526c125",
      "611b85b9b2b924245526c125",
      "611b85b9b2b924245526c125"
    ]
  }
]

Desired output would be all values from predictions arrays as an single array:

["611b85b9b2b924245526c125", "611b85b9b2b924245526c125", "611b85b9b2b924245526c125"]

Any ideas, pointers would be much appreciated. Obviously I am well aware it's trivial to transform current output to desired shape with JS but goal is to achieve final product with MongoDB Aggregation

Takis · Accepted Answer · 2021-08-19 22:01:01Z

2

$unwind all predictions, group by null(all collection as 1 group), and $push all members to array.

Test code here

db.collection.aggregate([
  {
    "$unwind": {
      "path": "$predictions"
    }
  },
  {
    "$group": {
      "_id": null,
      "predictions": {
        "$push": "$predictions"
      }
    }
  },
  {
    "$project": {
      "_id": 0
    }
  }
])

answered Aug 19, 2021 at 22:01

Takis

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1 Comment

Red Hood Over a year ago

Wow! I have no idea what just happened but it's actually works

Muhammad Mujtaba Shafique · Accepted Answer · 2021-08-19 21:13:00Z

0

As you want to skip your id column so you should use the $skip command of mongodb. It allows you to skips number of documents or any other condition as applied by you

answered Aug 19, 2021 at 21:13

Muhammad Mujtaba Shafique

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1 Comment

Red Hood Over a year ago

{$skip:"$_id"} gives error: MongoError: Argument to $skip must be a number not a string

Muhammad Mujtaba Shafique · Accepted Answer · 2021-08-19 21:37:18Z

0

If you want to conditionally remove some data from your output. You can use the following query

db.collection.aggregate( [ { $project : { "_id" : 0, "predictions" : 1 } } ] )

answered Aug 19, 2021 at 21:37

Muhammad Mujtaba Shafique

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1 Comment

Red Hood Over a year ago

I have already tried that but result is ``` [ { "Predictions": [ "611b85b9b2b924245526c125", "611b85b9b2b924245526c125", "611b85b9b2b924245526c125", "611b85b9b2b924245526c125" ] } ] ``` Still nested array with unnecessary keys instead of top level array with string items. I guess this is best I can get

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