MongoDB query to project a document as a nested object

Question

Hello Below I have specify the data which is to be used and get an output which I have mentioned below, Can you please help me on that as I tried alot but couldn't find a way.

//This is the data
let data = [
  {'name': 'A', 'parent': null},
  {'name': 'B', 'parent': null},
  {'name': 'C', 'parent': 'A'},
  {'name': 'D', 'parent': 'A'},
  {'name': 'E', 'parent': 'D'},
  {'name': 'F', 'parent': 'D'},
  {'name': 'G', 'parent': 'B'},
  {'name': 'H', 'parent': 'B'},
 ];

//And want the output like this by using mongodb aggregation
{
 "null": [
  {
   "A": [
    {
     "C": []
    },
    {
     "D": [
      {
       "E": []
      },
      {
       "F": []
      }
     ]
    }
   ]
  },
  {
   "B": [
    {
     "G": []
    },
    {
     "H": []
    }
   ]
  }
 ]
}

I have tried using graphlookup aggregation but couldn't find a way out of it.

Thanks for your help !

Can the depth be more than 3 or is it fixed as shown in sample output? — Dheemanth Bhat
– Dheemanth Bhat, Commented Mar 31, 2021 at 5:59
@ CreatorMind Check the updated answer. I will improve it in next edit. Can you confirm if the first draft is ok? — Dheemanth Bhat
– Dheemanth Bhat, Commented Mar 31, 2021 at 17:02

Dheemanth Bhat · Accepted Answer · 2021-03-31 17:01:33Z

Solution in Mongoplayground.

Try this:

db.collection.aggregate([
    {
        $lookup: {
            from: "collection",
            let: { name: "$name" },
            pipeline: [
                {
                    $match: {
                        $expr: { $eq: ["$parent", "$$name"] }
                    }
                },
                {
                    $lookup: {
                        from: "collection",
                        let: { name: "$name" },
                        pipeline: [
                            {
                                $match: {
                                    $expr: { $eq: ["$parent", "$$name"] }
                                }
                            },
                            {
                                $replaceRoot: {
                                    newRoot: {
                                        $arrayToObject: [[{ k: "$name", v: [] }]]
                                    }
                                }
                            }
                        ],
                        as: "children"
                    }
                },
                {
                    $replaceRoot: {
                        newRoot: {
                            $arrayToObject: [[{ k: "$name", v: "$children" }]]
                        }
                    }
                }
            ],
            as: "children"
        }
    },
    {
        $match: {
            children: { $ne: [] },
            parent: null
        }
    },
    {
        $group: {
            _id: "$parent",
            array: {
                $push: {
                    array: [{ k: "$name", v: "$children" }]
                }
            }
        }
    },
    {
        $addFields: {
            array: {
                $map: {
                    input: "$array",
                    as: "item",
                    in: { $arrayToObject: "$$item.array" }
                }
            }
        }
    },
    {
        $replaceRoot: {
            newRoot: { $arrayToObject: [[{ k: "null", v: "$array" }]] }
        }
    }
]);

Output:

Thanks alot ! As its bit lengthy stages but never mind it would help me for further process to cut it down.

Syed Mohib Uddin · Accepted Answer · 2021-03-31 05:46:03Z

0

You can do it this way.

  db.collection.aggregate([
  {
    $graphLookup: {
      from: "collection",
      startWith: "$name",
      connectFromField: "name",
      connectToField: "parent",
      as: "graph"
    }
  },
  {
    "$project": {
      name: 1,
      parent: 1
    }
  },
])

If you want to show it in array form you can create object and can use $objectToArray for ref check the link. $objectToArray

answered Mar 31, 2021 at 5:46

Syed Mohib Uddin

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1 Comment

Creator Mind Over a year ago

Hey! Thanks for your response , but the output is not same as I have mentioned though I I am not considering the array forms but the tree structure should be there as shown.

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