Instance methods not defined for Mongoose findOne() query result? - node.js

Question

My problem is that I can't call a model instance method on the document returned from a findOne() query, even though I can successfully call the instance method on the same document before I save it to the collection.
First, the usual require()s, followed by schema creation:
var express = require('express');
var router = express.Router();
const mongoose = require('mongoose');
const pwHash = require('password-hash');
const dbAddress = 'mongodb://localhost/18-652_Ramp_Up_Project';
const projectDb = mongoose.createConnection(dbAddress);
const exampleUserSchema = new mongoose.Schema({
username: String,
password: String,
});
// Instance methods of the User class:
exampleUserSchema.methods.checkPw = function(pwToAuthenticate) {
return pwHash.verify(pwToAuthenticate, this.password);
};
// Retrieve the document corresponding to the specified username.
exampleUserSchema.statics.findOneByUsername = function(username) {
return new Promise((resolve, reject) => {
projectDb.collection('exampleUsers').findOne(
{username: username},
(err, existingUser) => {
if (err) throw(err);
return resolve(existingUser ? existingUser : null);
}
);
});
};
// Constructor (through mongo);
const ExampleUser = projectDb.model('ExampleUser', exampleUserSchema, 'exampleUsers');
Now the interesting part: I can create a new user and call the instance method on it successfully:
// Create a test user and try out the
const hashedPw = pwHash.generate('fakePassword');
ExampleUser.create({username: 'fakeUsername', password: hashedPw}, (err, newUser) => {
console.log(newUser.checkPw('fakePassword')); // --> true
console.log(newUser.checkPw('password123')); // --> false
newUser.save((err) => {
console.log('New user ' + newUser.username + ' saved.'); // --> New user fakeUsername saved.
});
});
But when I try to call it on the document returned from a findOne() query, it's "not a function":
/* POST user login info. */
router.post('/login', (req, res, next) => {
// Try to look up the user in the database.
ExampleUser.findOneByUsername(req.body.username) // Static method works.
.then((existingUser) => {
console.log(existingUser);
if (existingUser && existingUser.checkPw(req.body.password)) { // Instance method doesn't.
console.log('Access granted.');
} else {
console.log('Access denied.');
}
});
});
module.exports = router;
This produces:
POST /example/login 200 22.587 ms - 14
{ _id: 5b3d592d8cd2ab9e27915380,
username: 'fakeUsername',
password: 'sha1$ea496f95$1$6a01df977cf204357444e263861041c622d816b6',
__v: 0 }
(node:40491) UnhandledPromiseRejectionWarning: TypeError: existingUser.checkPw is not a function
at ExampleUser.findOneByUsername.then (/Users/cameronhudson/Documents/GitHub/18-652_Ramp_Up_Project/routes/exampleUsers.js:52:42)
at process._tickCallback (internal/process/next_tick.js:68:7)