var kUrl = "file://jstests/noPassthrough/coll.bson";
var kMd = {url: kUrl};
var pipeline = [
    {$unionWith: {external: {urls: [kMd], storageType: "local"}}},
    {$match: {a: {$lt: 10}}},
    {$lookup: {
        from: {urls: [kMd], storageType: "local"},
        localField: "a",
        foreignField: "a",
        pipeline: [{$match: {a: {$lt: 5}}}],
        as: "j"
    }},
    {$unwind: "$j"}
];

db[1].aggregate(pipeline);