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$lookup & $unwind

$lookup & $unwind

Section titled “$lookup & $unwind”

$lookup joins documents from another collection. $unwind flattens arrays.

$lookup Basic Syntax

Section titled “$lookup Basic Syntax”
{
  $lookup: {
    from: "users",
    localField: "userId",
    foreignField: "_id",
    as: "user"
  }
}

Join Orders With Users

Section titled “Join Orders With Users”
db.orders.aggregate([
  {
    $lookup: {
      from: "users",
      localField: "userId",
      foreignField: "_id",
      as: "user"
    }
  }
])

The result contains user as an array:

{
  _id: ObjectId("..."),
  userId: ObjectId("..."),
  totalAmount: 2500,
  user: [
    { _id: ObjectId("..."), name: "Alice" }
  ]
}

$unwind

Section titled “$unwind”

$unwind converts each array element into a separate document.

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

Preserve Empty Arrays

Section titled “Preserve Empty Arrays”

Use this when a missing match should not remove the document.

{ $unwind: { path: "$user", preserveNullAndEmptyArrays: true } }

$lookup With Pipeline

Section titled “$lookup With Pipeline”

Use pipeline form when the join needs filters, projections, or multiple conditions.

db.users.aggregate([
  {
    $lookup: {
      from: "orders",
      let: { userId: "$_id" },
      pipeline: [
        { $match: { $expr: { $eq: ["$userId", "$$userId"] } } },
        { $match: { status: "completed" } },
        { $project: { totalAmount: 1, createdAt: 1 } }
      ],
      as: "completedOrders"
    }
  }
])

Common $unwind Use Case

Section titled “Common $unwind Use Case”

Flatten order items:

db.orders.aggregate([
  { $unwind: "$items" },
  {
    $group: {
      _id: "$items.productId",
      totalSold: { $sum: "$items.quantity" }
    }
  }
])