join()

const { MongoClient } = require("mongodb");
const client = new MongoClient("mongodb://localhost:27017");

async function connectToMongo() {
  try {
    await client.connect();
    console.log("Connected to MongoDB");
  } catch (err) {
    console.error(err);
  }
}

async function createCollections() {
  try {
    await connectToMongo();

    const db = client.db("test");

    const studentCollection = await db.createCollection("student");
    const courseCollection = await db.createCollection("course");

    // Insert some demo data into the collections
    await studentCollection.insertMany([
      { name: "olee", roll: 23 },
      { name: "john", roll: 24 },
      { name: "doe", roll: 25 },
    ]);

    await courseCollection.insertMany([
      { name: "Math", student_id: 23 },
      { name: "Science", student_id: 23 },
      { name: "English", student_id: 24 },
      { name: "History", student_id: 25 },
    ]);

    client.close();
  } catch (err) {
    console.error(err);
  }
}

createCollections();

async function joinCollections() {
  try {
    await connectToMongo();

    const db = client.db("test");

    const studentCollection = db.collection("student");
    const courseCollection = db.collection("course");

    // Join the two collections on the `student_id` field
    const cursor = await studentCollection
      .aggregate([
        {
          $lookup: {
            from: courseCollection.collectionName,
            localField: "_id",
            foreignField: "student_id",
            as: "courses",
          },
        },
      ])
      .find({});

    while (await cursor.hasNext()) {
      const document = await cursor.next();
      console.log("Document with joined data:");
      console.log(document);
    }

    client.close();
  } catch (err) {
    console.error(err);
  }
}

joinCollections();