MongoDB — Lookup

Basic Lookup

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

Parameters

Parameter	Description
from	Collection to join
localField	Field from input
foreignField	Field from joined
as	Output array field

Multiple Conditions

db.orders.aggregate([
  {
    $lookup: {
      from: "users",
      let: { userId: "$userId", status: "$status" },
      pipeline: [
        { $match: { $expr: { $eq: ["$_id", "$$userId"] } } }
      ],
      as: "user"
    }
  }
])

Unwind Result

db.orders.aggregate([
  { $lookup: { ... } },
  { $unwind: "$user" }
])

Mini Practice

1. Perform basic lookup
2. Use multiple conditions
3. Unwind lookup results
4. Combine with other stages

Up Next

Continue with Indexes — creating indexes.