bulkSave doesn't throw an error even if one of the submitted documents was not updated

[ksyd9821]

Prerequisites

• I have written a descriptive issue title

Mongoose version

8.x.x

Node.js version

20.11

MongoDB version

7.0

Operating system

None

Operating system version (i.e. 20.04, 11.3, 10)

No response

Issue

Hi all,

While troubleshooting we ran into some unexpected behavior of the bulkSave function.
If one of the submitted documents is outdated (has a lower version than the copy on the DB), this operation will not throw any error, even though the document won't be updated.
We expected it to behave similarly to the save function, which throws a VersionError in these cases.

Repro:

    public async foo() {
        const fooSchema = new mongoose.Schema({
            bar: { type: Number },
        });
        const model = mongoose.model("foo", fooSchema);
        await model.deleteMany();

        const foo = await model.create({
            bar: 0
        });

        // update 1
        foo.bar = 1;
        await foo.save();

        // parallel update
        const fooCopy = await model.findById(foo._id);
        fooCopy.bar = 99;
        await fooCopy.save();

        // update 2 - this should throw a version error
        foo.bar = 2;
        // await foo.save(); // throws a VersionError
        await model.bulkSave([foo]) // no error thrown, but update not applied

        // log result
        console.log(`bar: ${(await model.findById(foo._id)).bar}`); // logs 99
    }

Is this the expected behavior of bulkSave? Is there any possibility of making it fail if one of the documents has been already replaced by a newer version instead of returning silently?

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[vkarpov15]

As written, neither await foo.save() nor await model.bulkSave([foo]) throw a VersionError, and the final result is 2 not 99. In general, versioning in Mongoose only applies to array fields by default. However, the following script demonstrates the issue you're seeing (optimistic concurrency enabled)

const mongoose = require('mongoose');

require('util').inspect.defaultOptions.depth = 10;

mongoose.connect('mongodb://127.0.0.1:27017/mongoose_test');

void async function main() {
        const fooSchema = new mongoose.Schema({
            bar: { type: Number },
        }, { optimisticConcurrency: true });
        const model = mongoose.model("foo", fooSchema);
        await model.deleteMany();

        const foo = await model.create({
            bar: 0
        });

        // update 1
        foo.bar = 1;
        await foo.save();

        // parallel update
        const fooCopy = await model.findById(foo._id);
        fooCopy.bar = 99;
        await fooCopy.save();

        // update 2 - this should throw a version error
        foo.bar = 2;
        // await foo.save(); // throws a VersionError
        await model.bulkSave([foo]) // no error thrown, but update not applied

        // log result
        console.log(`bar: ${(await model.findById(foo._id)).bar}`); // logs 99
}();

[vkarpov15]

So long story short, there's no way for bulkSave() to know which documents failed to save due to concurrency issues. MongoDB bulkWrite() doesn't return how many documents each update operation updated. So if you bulkSave() 3 docs, and MongoDB server tells us that 2 docs matched, there's no way for Mongoose to know which 2 docs matched and which one didn't. All we get back from bulkWrite() is matchedCount, which contains the total number of documents matched by all the update operations.

@AbdelrahmanHafez @hasezoey what do you think about this one? There's a couple of options:

1. Make bulkSave() throw an error if matchedCount is 0: that way we know all documents failed to update
2. Make bulkSave() throw an error if matchedCount isn't equal to the number of documents being saved. In that case, we know at least one document wasn't found.
3. Keep the current behavior: bulkSave() never throws a DocumentNotFoundError or VersionError, but we should make a note in the docs that bulkSave() is unable to report versioning errors