Access mongoose parent document for default values in subdocument

I have a backend API for an Express/Mongo health tracking app.

Each user has an array of weighIns, subdocuments that contain a value, a unit, and the date recorded. If no unit is specified the unit defaults to 'lb'.

const WeighInSchema = new Schema({
  weight: {
    type: Number,
    required: 'A value is required',
  },
  unit: {
    type: String,
    default: 'lb',
  },
  date: {
    type: Date,
    default: Date.now,
  },
});

Each user also has a defaultUnit field, that can specify a default unit for that user. If that user posts a weighIn without specifying a unit, that weighIn should use the user's defaultUnit if present or else default to 'lb'.

const UserSchema = new Schema({
  email: {
    type: String,
    unique: true,
    lowercase: true,
    required: 'Email address is required',
    validate: [validateEmail, 'Please enter a valid email'],
  },
  password: {
    type: String,
  },
  weighIns: [WeighInSchema],
  defaultUnit: String,
});

Where is correct location for this logic?

I can easily do this in the create method of my WeighInsController, but this seems at best not best practice and at worst an anti-pattern.

// WeighInsController.js

export const create = function create(req, res, next) {
  const { user, body: { weight } } = req;
  const unit = req.body.unit || user.defaultUnit;
  const count = user.weighIns.push({
    weight,
    unit,
  });
  user.save((err) => {
    if (err) { return next(err); }
    res.json({ weighIn: user.weighIns[count - 1] });
  });
};

It doesn't seem possible to specify a reference to a parent document in a Mongoose schema, but I would think that a better bet would be in my pre('validate') middleware for the subdocument. I just can't see a way to reference the parent document in the subdocument middleware either.

NB: This answer does not work as I don't want to override all of the user's WeighIns' units, just when unspecified in the POST request.

Am I stuck doing this in my controller? I started with Rails so I have had 'fat models, skinny controllers' etched on my brain.

You can access the parent (User) from a sub-document (WeighIn) using the this.parent() function.

However, I'm not sure if it's possible to add a static to a sub-document, so that something like this would be possible:

user.weighIns.myCustomMethod(req.body)

Instead, you could create a method on the UserSchema, like addWeightIn:

UserSchema.methods.addWeightIn = function ({ weight, unit }) {
  this.weightIns.push({
    weight,
    unit: unit || this.defaultUnit
  })
}

Then just call the user.addWeightIn function within your controller and pass the req.body to it. This way, you get 'fat models, skinny controllers'.