Mongo Unwind child object array to root as plain array

Question

Suppose I have a simple collection like this:

[
  {
    Brands: [ 
                { BID: 1, Prods: [ { PID: 10 }, { PID: 11 } ] },
                { BID: 2, Prods: [ { PID: 20 }, { PID: 21 } ] }
           ]
  },
  {
    Brands: [
                { BID: 3, Prods: [ { PID: 30 }, { PID: 31 }, { PID: 32 } ] }
    ]
  }
]

I want to extract all unique PID from Prods, so the result would be:

[10,11,20,21,30,31,32]

I could get close to this doing this:

db.collection.aggregate([
  {
    $project: {
      "AllProds": {
        "$map": {
          "input": "$Brands.Prods",
          "as": "elem",
          "in": "$$elem.PID"
        }
      }
    }
  }
])

But the result is not a simple array, its a array of objects with the PID inside of it:

[
  {
    "AllProds": [
      [ 10, 11 ],
      [ 20, 21 ]
    ],
    "_id": ObjectId("5a934e000102030405000000")
  },
  {
    "AllProds": [
      [ 30, 31, 32 ]
    ],
    "_id": ObjectId("5a934e000102030405000001")
  }
]

Here is is the playground

Beingnin · Accepted Answer · 2020-12-10 02:34:57Z

1

You don't exactly require $map here. Just unwind the outer array, then the inner array and group them together

db.getCollection('SampleCollection').aggregate([
    {
        $unwind:"$Brands"
    },
    {
        $unwind:"$Brands.Prods"
    },
    {
        $group:
        {
            _id:null,
            PIDs: {$push:"$Brands.Prods.PID"}//use $addToSet instead of $push if you do not wish to allow duplicates
        }
    }    
])

edited Dec 10, 2020 at 2:34

answered Dec 10, 2020 at 2:27

Beingnin

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Comments

turivishal · Accepted Answer · 2020-12-10 05:35:28Z

$group by null and make Brands array
$project, $reduce to iterate loop of $Brands array, $reduce to iterate loop of inner array of brands and concat arrays using $concatArrays

db.collection.aggregate([
  {
    $group: {
      _id: null,
      Brands: { $push: "$Brands.Prods.PID" }
    }
  },
  {
    $project: {
      _id: 0,
      Brands: {
        $reduce: {
          input: "$Brands",
          initialValue: [],
          in: {
            $concatArrays: [
              "$$value",
              {
                $reduce: {
                  input: "$$this",
                  initialValue: [],
                  in: { $concatArrays: ["$$value", "$$this"] }
                }
              }
            ]
          }
        }
      }
    }
  }
])

Playground

ray · Accepted Answer · 2024-04-13 17:52:09Z

1

You can just do 2 more $unwind to flatten the arrays. Then use $group and $addToSet to get the result.

db.collection.aggregate([
  {
    $project: {
      "AllProds": {
        "$map": {
          "input": "$Brands.Prods",
          "as": "elem",
          "in": "$$elem.PID"
        }
      }
    }
  },
  {
    "$unwind": "$AllProds"
  },
  {
    "$unwind": "$AllProds"
  },
  {
    "$group": {
      "_id": null,
      "uniquePids": {
        "$addToSet": "$AllProds"
      }
    }
  }
])

Here is the mongo playground for your reference.

edited Apr 13, 2024 at 17:52

answered Dec 10, 2020 at 2:16

ray

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