db.localColl.drop();
db.foreignColl.drop();

db.adminCommand({setParameter: 1, internalEnableJoinOptimization: true});

db.localColl.insertMany([
  { _id: 1, foo: [10, 20], localPayload: "L1" },
  { _id: 2, foo: [30, 40], localPayload: "L2" },
  { _id: 3, foo: [50, 60], localPayload: "L3" }
]);
db.foreignColl.insertMany([
  { _id: "a", bar: [10, 99], foreignPayload: "F1" },
  { _id: "b", bar: [30, 88], foreignPayload: "F2" },
  { _id: "c", bar: [70, 77], foreignPayload: "F3" }
]);

db.localColl.createIndex({ "foo.0": 1 });
db.foreignColl.createIndex({ "bar.0": 1 });

const pipeline = [
  {
    $lookup: {
      from: "foreignColl",
      localField: "foo.0",
      foreignField: "bar.0",
      as: "joined"
    }
  },
  { $unwind: "$joined" }
];
db.localColl.aggregate(pipeline).forEach(doc => printjson(doc));