mongo aggregate,group の検証

add.txt

// add data
db.book_groups.insertMany([
	{ dept_name: "dept-1", name: "group-1" },
	{ dept_name: "dept-1", name: "group-2" },
	{ dept_name: "dept-2", name: "group-11" },
]);

//
db.book_groups.aggregate(
{
    $group : { _id: "$dept_name" }
});

db.book_groups.aggregate(
{
    $group : { _id: "$dept_name", total : { $sum : 1 } }
});

t3_group.js

//
// mongo aggregate,group の検証
var ObjectID = require('mongodb').ObjectID;
const MongoClient = require('mongodb').MongoClient;

//
async function join_test(){
    let client = await MongoClient.connect( "mongodb://localhost:27017" );
    const db = client.db("db1");
    var book_groups= []
    await db.collection("book_groups").aggregate([
    { $group : { _id: "$dept_name" ,total : { $sum : 1 } } },
    ]).toArray().then((docs) => {
//        console.log(docs);
        book_groups = docs;
    })
    .catch((err) => {
        console.log(err);
    })
    .then(() => {
        client.close();
    });
console.log( book_groups );
    book_groups.forEach(function(item){
//        console.log( item._id  );
    })
}

// main
join_test();