Mongoose discriminator is ignoring an extra field

So, I have a Category model with three discriminators on it. One of them, "Allowance" has a new Schema for an extra field. However, when a new Category is saved to the DB, that field will not save, it is being thrown out by Mongoose.

const CategorySchema = new mongoose.Schema({
    name: {
        type: String,
        required: [true, "MUST HAVE A NAME"],
        validate: {
            validator: isSanitary,
            message: "NAME CONTAINS ILLEGAL CHARACTERS"
        }
    },
    amount: {
        type: Number,
        required: [true, "MUST CONTAIN AN AMOUNT"]
    },
    removed: {
        type: Boolean,
        default: false
    }
}, {discriminatorKey: "kind"});
const Category = mongoose.model("Category", CategorySchema);

Category.discriminator("Income", new mongoose.Schema());

Category.discriminator("Bill", new mongoose.Schema());

Category.discriminator("Allowance",
    new mongoose.Schema({
        isPercent: {
            type: Boolean,
            required: true
        }
    })
);

module.exports = {
    Income: Income,
    Bill: Bill,
    Allowance: Allowance,
    CategorySchema: CategorySchema
};

Requiring the model:

const {Income, Bill, Allowance} = require("../models/category.js");

Code for creating a new Category:

/*
    POST: create a new Category
    req.body = {
        account: String (Account id)
        name: String
        amount: Number
        kind: String (Income, Bill, Allowance)
        isPercent: Boolean
    }
    response: Category
    */
    createCategory: function(req, res){
        let category = {
            name: req.body.name,
            amount: req.body.amount,
            removed: false
        };

        switch(req.body.kind){
            case "Income":
                category = new Income(category);
                break;
            case "Bill":
                category = new Bill(category);
                break;
            case "Allowance":
                category = new Allowance(category);
                category.isPercent = req.body.isPercent;
                break;
        }

        res.locals.user.accounts.id(req.body.account).categories.push(category);

        res.locals.user.save()
            .then((user)=>{
                return res.json(category);
            })
            .catch((err)=>{
                console.error(err);
                return res.json("ERROR: unable to create new category");
            });
    },

When a new "Allowance" is saved, it saves fine, except that isPercent is not saved to the DB. This was working previously but then stopped when I made a tiny change. The only change is that I started exporting the Schema as well as the model. I have no idea how to even debug this issue.

Thanks for any help.

The issue is that you are treating Category model the same as the Allowance category discriminator model. You cannot save isPercent to Category, only Allowance. Notice in the docs that when they want to create a discriminator model, they don't use the base, they use the discriminator. Try instead exporting the discriminator models as well:

const Income = Category.discriminator("Income", new mongoose.Schema());
const Bill = Category.discriminator("Bill", new mongoose.Schema());
const Allowance = Category.discriminator("Allowance",
    new mongoose.Schema({
        isPercent: {
            type: Boolean,
            required: true
        }
    })
);

module.exports = {
    CategorySchema,
    Category,
    Income,
    Bill,
    Allowance,
};

Then you can update the logic in your route handler to create an Allowance or Category conditionally:

const { Allowance, Category } = require("../models/category");
 
// ...    

// Get correct model based on req.body data
const Model = req.body.kind === "Allowance" ? Allowance : Category;
let category = new Model({
  name: req.body.name,
  amount: req.body.amount,
  kind: req.body.kind,
  isPercent: req.body.kind === "Allowance" ? req.body.isPercent : undefined,
});

Hopefully that helps!