Access mongoose parent document for default values in subdocument - mongodb

Question

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.

Answer 1

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'.