CODE EXAMPLE FOR JAVASCRIPT

mongoose

// schema creation...

let modal;
SchemaVariable.pre('save', function(next) {
  if (this.isNew) {
    if (!modal) {
      modal = mongoose.model('collectionname');
    }
    modal.find({})
      .then((entries) => {
        this._id = entries.length + 1;
        next();
      })
  }
});