如何从 Javascript 中的集合数组中提取键的值

Question

I want to get the values of the keys in an array from a series of collection. Please find the snippet of code and expected output. I would be grateful if you can help me with what should go into the aggregate phase to get the desired output. Please note there will be more than 5000 collections

db.collection1.insertMany([{
    item: "journal",
    qty: 25,
    tags: blank
  },
  {
    item: "mat",
    qty: 85,
    tags: gray
  },
  {
    item: "mousepad",
    qty: 25,
    tags: gel
  }
])

db.collection2.insertMany([{
    abc: "paplu",
    qiity: 01,
    thugs: red
  },
  {
    abc: "mat",
    qiity: 85,
    thugs: gray
  },
  {
    abc: "mousepad",
    qiity: 25,
    thugs: gel
  }
])

var a = ["collection1", "collection2"];


for (var i = 0; i < a.length; i++) {
db[a[i]].aggregate([])};



Expected Output:
collection1
{
item : ["journal","mat","mousepad"],
qty : [25,85,25],
tags : [blank,gray,gel]
}
collection2
{
abc : ["paplu","mat","mousepad"],
qiity : [01,85,25],
thugs : [red,gray, gel]
}





Please note, I'm trying to achieve this using MongoDB/JavaScript

It would be great if someone can help me with this!

Answer 1

You can start with $objectToArray to get keys from ROOT object into an array. Then you can run $unwind and $group by null with $addToSet to get single document which contains unique key names from entire collection. In the last step you need to convert an array back to single document using $map , $arrayToObject and $replaceRoot :

db.collection.aggregate([
    {
        $project: { kv: { $objectToArray: "$$ROOT" } }
    },
    {   $unwind: "$kv" },
    {
        $group: {
            _id: null,
            keys: { $addToSet: "$kv.k" }
        }
    },
    {
        $replaceRoot: {
            newRoot: {
                $arrayToObject: {
                    $map: { input: "$keys", in: [ "$$this", 1 ] }
                }
            }
        }
    }
])

Mongo Playground