find.js

const choiceId = '588776235f5cab3f9af45e81'

pollModel.findOneAndUpdate(
  { 'choices._id': choiceId },
  { $inc: { 'choices.$.votes': 1 } },
  { new: true }
)

/**
{
  "data": {
    "message": "OK",
    "poll": {
      "_id": "588776235f5cab3f9af45e7f",
      "title": "TEST",
      "url": "wgrcx",
      "choices": [
        {
          "title": "Test API Choice 1",
          "_id": "588776235f5cab3f9af45e81",
          "votes": 15
        },
        {
          "title": "Test API Choice 2",
          "_id": "588776235f5cab3f9af45e80",
          "votes": 0
        }
      ]
    }
  }
}
*/

schema.js

const PollSchema = new Schema({
  title: String,
  choices: [
    {
      title: String,
      votes: { type: Number, default: 0 }
    }
  ],
  date: { type: Date, default: Date.now },
  url: String,
})