Example of $lookup vs populate()

student.service.js

/*
 * 3 Extra DB quries will be made
 */
StudentSchema.find()
  .populate("class")
  .populate("department")
  .populate("teacher");

/*
 * No extra queries, Less load on your Node.js server
 */
StudentSchema.aggregate([
  {
    $lookup: {
      from: "classes",
      localField: "class",
      foreignField: "_id",
      as: "class",
    },
  },
  { $unwind: "$class" },
  {
    $lookup: {
      from: "departments",
      localField: "department",
      foreignField: "_id",
      as: "department",
    },
  },
  { $unwind: "$department" },
  {
    $lookup: {
      from: "teachers",
      localField: "teacher",
      foreignField: "_id",
      as: "teacher",
    },
  },
  { $unwind: "$teacher" },
]);