MongoDB: add object to subarray if not exists

Question

I searched many questions here and other articles on the web, but they all seem to describe somehow different cases from what I have at hand.

I have User schema:

{
    username: { type: String },
    lessons: [
        {
            lesson: { type: String },
            result: { type: String }
        }
    ]
}

I want to add new element into lessons or skip, if there is already one with same values, therefore I use addToSet:

const dbUser = await User.findOne({ username })
dbUser.lessons.addToSet({ lesson, result: JSON.stringify(result) })
await dbUser.save()

However it makes what seems to be duplicates:

// first run
[
  {
    _id: 60c80418f2bcfe5fb8f501c1,
    lesson: '60c79d81cf1f57221c05fdac',
    result: '{"correct":2,"total":2}'
  }
]

// second run
[
  {
    _id: 60c80418f2bcfe5fb8f501c1,
    lesson: '60c79d81cf1f57221c05fdac',
    result: '{"correct":2,"total":2}'
  },
  {
    _id: 60c80470f2bcfe5fb8f501c2,
    lesson: '60c79d81cf1f57221c05fdac',
    result: '{"correct":2,"total":2}'
  }
]

At this point I see that it adds _id and thus treats them as different entries (while they are identical).

What is my mistake and what should I do in order to fix it? I can change lessons structure or change query - whatever is easier to implement.

You can check for the lesson and result values (along with username you are already filtering on) in the query before the update (save). — prasad_
– prasad_, Commented Jun 15, 2021 at 2:24
@prasad_ so what you suggest is simply if (await User.findOne({ username }, { lesson })) condition (let's ignore result)? — Arctomachine
– Arctomachine, Commented Jun 15, 2021 at 2:39
Something like this: mongoose “Find” with multiple conditions. The lesson condition can be coded as "lessons.lesson": "math", for example. — prasad_
– prasad_, Commented Jun 15, 2021 at 3:06

Duy Quoc · Accepted Answer · 2021-06-15 03:06:14Z

1

You can create sub-documents avoid _id. Just add _id: false to your subdocument declaration.

const userSchema = new Schema({
  username: { type: String },
  lessons: [
    {
      _id: false,
      lesson: { type: String },
      result: { type: String }
    }
  ]
});

This will prevent the creation of an _id field in your subdoc, and you can add a new element to the lesson or skip it with the addToSet operator as you did.

edited Jun 15, 2021 at 3:06

answered Jun 15, 2021 at 3:00

Duy Quoc

1811 silver badge9 bronze badges

Sign up to request clarification or add additional context in comments.

1 Comment

Arctomachine Over a year ago

It works now. Without _id it finally compares them right.

Collectives™ on Stack Overflow

MongoDB: add object to subarray if not exists

1 Answer 1

1 Comment

Your Answer

Linked

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

1 Comment

Your Answer

Sign up or log in

Post as a guest

Linked

Related