Mongoose update conditions matching wrong object

Question

Model Schema

const PollSchema = new Schema({

  title: { type: String, trim: true, required: true },

  choices: [

    {

      title: { type: String, trim: true, required: true },

      votes: { type: Number, default: 0, min: 0 },

    }

  ],

  date: { type: Date, default: Date.now },

  url: { type: String, required: true, unique: true },

})

Update call

async vote (pollId, choiceId, unvoteId = '') {

  try {

    await pollModel.update(

      { '_id': pollId, 'choices._id': choiceId },

      { $inc: { 'choices.$.votes': 1 } },

    )



    if (unvoteId) {

      await pollModel.update(

        {

          "$and": [

            { '_id': pollId }, 

            { 'choices._id': unvoteId },

            { 'choices.votes': { $gt: 0 } }

          ],

        },

        { $inc: { 'choices.$.votes': -1 } },

      )

    }



    return await pollModel.findById(pollId)

  } catch (e) {

    throw new ApiError(

      500, 'Error: Poll:Vote', e

    )

  }

}

I have been trying a plethora of combinations trying to get this to work. Voting +1 works as intended, but when trying to -1, the query conditions are not properly matched. I have tried

$and

,

$elemMatch

, plain object with the 3 conditions (according to the docs this is sufficient and implicitly means

and

too.

Whenever I send through an unvoteId, no matter which

_id

I choose in the array, it will always match the FIRST element that has

$gt 0

votes. It is as if

choices._id

is completely ignored, and as soon as it meets a choice that has > 0 votes, it returns that for the

$

positional param.

Is this intended? I assumed

$and

would only match if all 3 conditions were satisfied.

What I am trying to do is update the votes atomically using

$inc

, while also ensuring that when someone votes, they cannot bring the value below 0. As the mongoose validators do not get run during updates, I am trying to validate this via the query itself.

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Answer 1

Try something like this.

pollModel.update({
    _id: pollId,
    choices: {
        $elemMatch: {
            _id: unvoteId,
            votes: {
                $gt: 0
            }
        }
    }
}, {
    $inc: {
        "choices.$.votes": -1
    }
})

Mongoose update conditions matching wrong object

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