Mongoose discriminators don't work?

CrankRight.js

const mongoose = require( "mongoose" );
const Part = require( "./Part" );

const options = { discriminatorKey: "kind" };
const schema = new mongoose.Schema({
	techincal: {
		bcd: Number,                      // mm
		bolts: Number,
		bbInterface: String,              // e.g. "square-taper-jis"
		bbLength: Number,                 // mm
		chainline: Number,                // mm
		crankLenght: Number               // mm
	}
}, options );

// Export the model
module.exports = Part.discriminator( "CrankRight", schema );

Part.js

const mongoose = require( "mongoose" );

const options = { discriminatorKey: "kind" };
const schema = new mongoose.Schema({
	general: {
		brand: String,
		group: String,
		name: String,
		years: [ Number ],
		productID: String,
		stock: Boolean,
		divisible: Boolean,
		includes: [ String ]
	},
	physical: {
		colors: [ String ],
		finishes: [ String ],
		materials: [ String ],
		markings: [ String ],
		weight: Number,
		condition: Number
	}
}, options );

// Export the model
module.exports = mongoose.model( "Part", schema );

server.js

const express = require( "express" );
const mongoose = require( "mongoose" );
const Part = require( "./models/Part" );
const CrankRight = require( "./models/CrankRight" );

mongoose.connect( "mongodb://localhost:27017/parts" );
const connection = mongoose.connection;
connection.on( "error", console.error.bind( console, "connection error" ) );
connection.once( "open", () => {
	console.log( "Connection succeeded" );
});

const app = express();

app.use( require( "morgan" )( "combined" ) );
app.use( require( "body-parser" ).json() );
app.use( require( "cors" )() );

app.listen( process.env.PORT || 8081 );

app.post( "/parts", ( request, response ) => {

	/* Example request.body
	{
		kind: "CrankRight"
		general: {
			brand: "Shimano",
			group: "Dura-Ace",
			name: "9100"
		},
		physical: {
			colors: [],
			materials: [],
			markings: [],
			finishes: []
		},
		technical: {
			bcd: 130
		}
	}
	*/
	let part;

	// Obviously there's a better way to determine which model to use but this works for the gist:
	if ( request.body.kind === "CrankRight" ) {
		console.log( "Creating a crank right" );
		part = new CrankRight( request.body );
	} else {
		console.log( "Creating a generic part" );
		part = new models.Part( request.body );
	}
	console.log( "PART CREATED IS:", part );
	
	// At this point, part does not contain the technical info, nor does it contain a __t key. It does contain "kind" though.
	part.save( ( error ) => {
		if ( error ) {
			console.log( error );
		}
		response.send( part );
	});
});

For some reason the technical attributes are not included in the saved document.