Replace `_id` with string `id` and remove `__v` when calling `toObject()` in Mongoose.

transform.js

if (!MySchema.options.toObject) {
  PlanSchema.options.toObject = {};
}

MySchema.options.toObject.transform =  function(doc, ret) {
  // Set the id from the retrun object value which will be a string.
  ret.id = ret._id;

  delete ret._id;
  delete ret.__v;
};

// The resulting `id` field will be something like "532263ad59a6316d59000004".