Example of $lookup with pipeline stages

user.service.js

/*
 * Traditional $lookup without pipeline
 */

UserSchema.aggregate([
  {
    $lookup: {
      from: "departments",
      localField: "department",
      foreignField: "_id",
      as: "department",
    },
  },
]);

/*
 * Output:
 * Whole department document has been populated
 * We can reduce the number of populated fields to save memory usage
 */

// [
//   {
//     _id: "62694214b0f6d30477dbe911",
//     firstName: "Navanath",
//     lastName: "Jadhav",
//     email: "navanath@everblogs.com",
//     role: "Admin",
//     department: [{
//       _id: "62b315309c9865676e1f2cf1",
//       __v: 0,
//       name: "Engineering",
//       createdAt: "2022-06-22T18:37:07+05:30",
//       updatedAt: "2022-06-22T18:37:07+05:30",
//       building: "A",
//       budget: 10000,
//     }],
//   }
// ];

/*
 * $lookup with pipeline stages
 * We have used $project pipeline stage in $lookup and it allowed us to select only necessary fields
 */

UserSchema.aggregate([
  {
    $lookup: {
      from: "departments",
      let: {
        department_id: "$department",
      },
      pipeline: [
        {
          $match: {
            $expr: {
              $and: [{ $eq: ["$_id", "$$department_id"] }],
            },
          },
        },
        {
          $project: {
            name: 1,
            building: 1,
            budget: 1,
          },
        },
      ],
      as: "department",
    },
  },
]);

/*
 * Output:
 * Only necessary fields have been populated as specified in $project
 * We have reduced the memory consumption here
 */

// [
//   {
//     _id: "62694214b0f6d30477dbe911",
//     firstName: "Navanath",
//     lastName: "Jadhav",
//     email: "navanath@everblogs.com",
//     role: "Admin",
//     department: [{
//       name: "Engineering",
//       building: "A",
//       budget: 10000,
//     }],
//   },
// ];