How to display only the keys from nested MongoDB documents?

Let us create a collection with documents −

> db.demo740.insertOne({
...    "details":
...    [
...       {
...          Name:"Chris",
...          Age:21,
...          CountryName:"US"
...       },
...       {
...          Name:"Bob",
...          Age:20,
...          CountryName:"UK",
...          isMarried:true
...       }
...    ]
... });
{
   "acknowledged" : true,
   "insertedId" : ObjectId("5ead700c57bb72a10bcf066d")
}

Display all documents from a collection with the help of find() method −

> db.demo740.find();

This will produce the following output −

"_id" : ObjectId("5ead700c57bb72a10bcf066d"), "details" : [ { "Name" : "Chris", "Age" : 21, "CountryName" : "US" }, { "Name" : "Bob", "Age" : 20, "CountryName" : "UK", "isMarried" : true } ] }

Following is the query to get keys of nested MongoDB documents −

> db.demo740.aggregate([
...    {
...       $project: {
...          ListKeys: {
...             $reduce: {
...                input: "$details",
...                initialValue: [],
...                in: {
...                   $concatArrays: [
...                      "$$value",
...                      {
...                         $map: {
...                            input: {
...                               $objectToArray: "$$this"
...                            },
...                            in: "$$this.k"
...                         }
...                      }
...                   ]
...                }
...             }
...          }
...       }
...    }
... ]).pretty()

This will produce the following output −

{
   "_id" : ObjectId("5ead700c57bb72a10bcf066d"),
   "ListKeys" : [
      "Name",
      "Age",
      "CountryName",
      "Name",
      "Age",
      "CountryName",
      "isMarried"
   ]
}