Aggregate by country, state and city in a MongoDB collection with multiple documents

Aggregation operations group values from multiple documents together, and can perform a variety of operations on the grouped data to return a single result.

To aggregate in MongoDB, use aggregate(). Let us create a collection with documents −

> db.demo620.insertOne({"Country":"IND","City":"Delhi",state:"Delhi"});
{
   "acknowledged" : true,
   "insertedId" : ObjectId("5e9a8de96c954c74be91e6a1")
}
> db.demo620.insertOne({"Country":"IND","City":"Bangalore",state:"Karnataka"});
{
   "acknowledged" : true,
   "insertedId" : ObjectId("5e9a8e336c954c74be91e6a3")
}
> db.demo620.insertOne({"Country":"IND","City":"Mumbai",state:"Maharashtra"});
{
   "acknowledged" : true,
   "insertedId" : ObjectId("5e9a8e636c954c74be91e6a4")
}

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

> db.demo620.find();

This will produce the following output −

{ "_id" : ObjectId("5e9a8de96c954c74be91e6a1"), "Country" : "IND", "City" : "Delhi", "state" : "Delhi" }
{ "_id" : ObjectId("5e9a8e336c954c74be91e6a3"), "Country" : "IND", "City" : "Bangalore", "state" : "Karnataka" }
{ "_id" : ObjectId("5e9a8e636c954c74be91e6a4"), "Country" : "IND", "City" : "Mumbai", "state" : "Maharashtra" }

Following is the query to aggregate by country, state and city −

> db.demo620.aggregate([
...    { "$group": {
...       "_id": {
...          "Country": "$Country",
...          "state": "$state"
...       },
...       "City": {
...          "$addToSet": {
...             "City": "$City"
...          }
...       }
...    }},
...    { "$group": {
...       "_id": "$_id.Country",
...       "states": {
...          "$addToSet": {
...             "state": "$_id.state",
...             "City": "$City"
...          }
...       }
...    }}
... ]).pretty();

This will produce the following output −

{
   "_id" : "IND",
   "states" : [
      {
         "state" : "Delhi",
         "City" : [
            {
               "City" : "Delhi"
            }
         ]
      },
      {
         "state" : "Maharashtra",
         "City" : [
            {
                  "City" : "Mumbai"
            }
         ]
      },
      {
         "state" : "Karnataka",
         "City" : [
            {
               "City" : "Bangalore"
            }
         ]
      }
   ]
}