$unwind after $lookup

Because $lookup always hands back an array, the stage that follows it is almost always $unwind. This pairing is common enough to be treated as a single idiom.

Classic pattern:

db.orders.aggregate([
  {
    $lookup: {
      from: "users",
      localField: "userId",
      foreignField: "_id",
      as: "user"
    }
  },

  {
    $unwind: "$user"
  }
])

Now:

{
  _id: 1,
  userId: 101,
  amount: 1000,

  user: {
    _id: 101,
    name: "Denish"
  }
}

This is the common pattern:

$lookup
    ↓
Returns array

$unwind
    ↓
Turns array into object

For one-to-one or many-to-one relationships, this is very common.