how to make a field unique in the subdocument in mongoose code example

Example 1: mongoose unique field

var SimSchema = new Schema({
    msisdn     : { type : String , unique : true, required : true, dropDups: true },
    imsi       : { type : String , unique : true, required : true, dropDups: true },
    status     : { type : Boolean, default: true},
    signal     : { type : Number },
    probe_name : { type:  String , required : true }
});

Example 2: mongoose unique subdocument

Model.update({
  _id: '576328595b2880f831413b92',
  'subdocs.name': {
    $ne: 'Unique Name'
  }
}, {
  $push: {
    subdocs: {
      name: 'Unique Name'
    }
  }
}).then((raw) => {
  // check raw.nModified value
}).catch(next);