The pipeline form of $lookup allows filtering and reshaping foreign documents. let passes values from the current document into the foreign pipeline. Reference these variables using $$name; use $expr when a $match compares expressions or variables.

Use $match, $project, $sort, and $limit inside the foreign pipeline as appropriate. Correlated lookups can be powerful but should be tested with representative data. Index eligibility depends on the expression and query shape.

db.orders.aggregate([
  {
    $lookup: {
      from: "customers",
      let: { orderCustomerId: "$customerId" },
      pipeline: [
        { $match: { $expr: { $eq: ["$_id", "$$orderCustomerId"] } } },
        { $project: { name: 1, city: 1, tier: 1 } },
        { $limit: 1 }
      ],
      as: "customer"
    }
  }
]);