mongodb/jstests/change_streams/does_not_implicitly_create_database.js

/**
 * Tests that change streams can be opened on a namespace before the collection or database has been
 * created, and will not implicitly create either.
 */

import {ChangeStreamTest} from "jstests/libs/query/change_stream_util.js";

// Ensure that the test DB does not exist.
const testDB = db.getSiblingDB(jsTestName());
assert.commandWorked(testDB.dropDatabase());

let dbList = assert.commandWorked(
    db.adminCommand({listDatabases: 1, nameOnly: true, filter: {name: testDB.getName()}}));
assert.docEq([], dbList.databases);

const collName = "test";

// Start a new $changeStream on the non-existent db.
const cst = new ChangeStreamTest(testDB);
const changeStreamCursor =
    cst.startWatchingChanges({pipeline: [{$changeStream: {}}], collection: collName});

// Confirm that a $changeStream cursor has been opened on the namespace.
assert.gt(changeStreamCursor.id, 0);

// Confirm that the database has not been implicitly created.
dbList = assert.commandWorked(
    db.adminCommand({listDatabases: 1, nameOnly: true, filter: {name: testDB.getName()}}));
assert.docEq([], dbList.databases);

// Confirm that a non-$changeStream aggregation on the non-existent database returns an empty
// cursor.
const nonCsCmdRes = assert.commandWorked(
    testDB.runCommand({aggregate: collName, pipeline: [{$match: {}}], cursor: {}}));
assert.docEq([], nonCsCmdRes.cursor.firstBatch);
assert.eq(nonCsCmdRes.cursor.id, 0);

// Now perform some writes into the collection...
assert.commandWorked(testDB[collName].insert({_id: 1}));
assert.commandWorked(testDB[collName].insert({_id: 2}));
assert.commandWorked(testDB[collName].update({_id: 1}, {$set: {updated: true}}));
assert.commandWorked(testDB[collName].remove({_id: 2}));

// ... confirm that the database has been created...
dbList = assert.commandWorked(
    db.adminCommand({listDatabases: 1, nameOnly: true, filter: {name: testDB.getName()}}));
assert.docEq([{name: testDB.getName()}], dbList.databases);

// ... and verify that the changes are observed by the stream.
const expectedChanges = [
    {
        documentKey: {_id: 1},
        fullDocument: {_id: 1},
        ns: {db: testDB.getName(), coll: collName},
        operationType: "insert"
    },
    {
        documentKey: {_id: 2},
        fullDocument: {_id: 2},
        ns: {db: testDB.getName(), coll: collName},
        operationType: "insert"
    },
    {
        documentKey: {_id: 1},
        ns: {db: testDB.getName(), coll: collName},
        updateDescription: {removedFields: [], updatedFields: {updated: true}, truncatedArrays: []},
        operationType: "update"
    },
    {documentKey: {_id: 2}, ns: {db: testDB.getName(), coll: collName}, operationType: "delete"},
];

cst.assertNextChangesEqual({cursor: changeStreamCursor, expectedChanges: expectedChanges});
cst.cleanUp();