Trimming the joined document

A join drags the entire matched document into your result, which is rarely what you want to return. $project after the join is how you keep the two fields you actually need.

Question:

Return each completed order with the user’s name.

db.orders.aggregate([
  {
    $match: {
      status: "completed"
    }
  },

  {
    $lookup: {
      from: "users",
      localField: "userId",
      foreignField: "_id",
      as: "user"
    }
  },

  {
    $unwind: "$user"
  },

  {
    $project: {
      _id: 1,
      amount: 1,
      userName: "$user.name"
    }
  }
])

Read it:

Find completed orders
        ↓
Join the user
        ↓
Convert user[] → user
        ↓
Return only what I need