mongo/jstests/aggregation/sources/replaceRoot/use_cases.js

// Basic integration tests for $replaceRoot and its alias $replaceWith.
const coll = db.replaceWith_use_cases;
coll.drop();

assert.commandWorked(
    coll.insert([
        {
            _id: 0,
            comments: [
                {user_id: "x", comment: "foo"},
                {user_id: "y", comment: "bar"},
            ],
        },
        {_id: 1, comments: [{user_id: "y", comment: "bar again"}]},
    ]),
);

// Test computing the most frequent commenters using $replaceRoot.
let pipeline = [{$unwind: "$comments"}, {$replaceRoot: {newRoot: "$comments"}}, {$sortByCount: "$user_id"}];
const expectedResults = [
    {_id: "y", count: 2},
    {_id: "x", count: 1},
];
assert.eq(coll.aggregate(pipeline).toArray(), expectedResults);

// Test the same thing but using the $replaceWith alias.
pipeline = [{$unwind: "$comments"}, {$replaceWith: "$comments"}, {$sortByCount: "$user_id"}];
assert.eq(coll.aggregate(pipeline).toArray(), expectedResults);