$addToSet misbehave - missing docs/information about behavior

[dor-benatia]

i've stumbled across a weird problem with mongoose $addToSet in patch function,

i am running this code :

const y = await model.updateOne(
    { _id: '61e0327e3889426f21dbbe25' },
    {
        $addToSet: { links: ['test1', 'test2', 'test3'] },
    },
);

when looking at the object in the db i see links: ['a','b',test1', 'test2', 'test3' ] which correspond to $addToSet with $each operator as explained in these docs: https://docs.mongodb.com/manual/reference/operator/update/addToSet/#value-to-add-is-an-array

which should of added this array as single element.

I am seeing in the code the corresponding line that caused that : lib/types/array/ArrayWrapper.js line 82.

The thing is that it is written no-where in the docs, not even a one liner.

maybe i am missing something ?

mongoose version: 6.0.13

[IslandRhythms]

const mongoose = require('mongoose');

const testSchema = new mongoose.Schema({
    name: []
});

const Test = mongoose.model('Test', testSchema);

async function run() {
await mongoose.connect('mongodb://localhost:27017');
await mongoose.connection.dropDatabase();

const entry = await Test.create({
    name: [1,2,3,4,5]
});

console.log(entry);

await Test.findOneAndUpdate({_id: entry._id}, {$addToSet: {name: [6,7,8,9,0]}});
console.log(await Test.find());
}

run();

[dor-benatia]

@IslandRhythms - this is exactly the code the reveal the problem - thank you.

[twhy]

What if there's code mistakenly rely on this misbehavior?

[vkarpov15]

Right now, the way that Mongoose handles this case is that it adds $each unless you're using $addToSet on a nested array path. For example, the below code doesn't add $each because name is a nested array.

const mongoose = require('mongoose');

mongoose.set('debug', true);

const testSchema = new mongoose.Schema({
    name: [[]] // <-- Array of arrays of any type
});

const Test = mongoose.model('Test', testSchema);

async function run() {
  await mongoose.connect('mongodb://localhost:27017');
  await mongoose.connection.dropDatabase();

  const entry = await Test.create({
    name: [1,2,3,4,5]
  });

  console.log(entry);

  await Test.findOneAndUpdate({_id: entry._id}, {$addToSet: {name: [6,7,8,9,0]}});
  console.log(await Test.find());
}

run();

We'll avoid adding $each for mixed arrays for v6.2.1 👍

[dor-benatia]

@twhy - i think it will appear as a breaking change and if you choose to update to version 6.2.1 you should handle that since it is obviously a bug.

[twhy]

@dor-benatia Agree.