How to automatically create a required field in mongoose - node.js

Question

I have a mongoose schema that looks like this:
var userSchema = new Schema({
username: {type: String, required: true, index: {unique: true}},
usernameCanonical: {type: String, required: true, index: {unique: true}}
});
userSchema.pre("save", function () {
this.usernameCanonical = this.username.toLowerCase();
return next();
});
I want to be able to create new users by only entering a username, and let usernameCanonical get generated by the model automatically.
var user = new User({
username: "EXAMPLE_USERNAME"
});
user.save()
When I try to do this I get a validation error from mongoose saying that usernameCanonical is required.
Path `usernameCanonical` is required.
The problem seems to be that the pre-save hooks get called after validation. I don't want to have to manually add a canonical username every time I save a new user. I also don't want to remove the required option from the schema.
Is there some way to get a mongoose model to automatically generate a required field? Adding a default value to the usernameCanonical field in the schema seems to prevent the validation error, but it feels like a hack.

Answer 1

As levi mentioned, you should use the validate() hook:
Save/Validate Hooks
Check this working example based on your code:
const mongoose = require('mongoose');
const Schema = mongoose.Schema;
const userSchema = new Schema({
username: {type: String, required: true, index: {unique: true}},
usernameCanonical: {type: String, required: true, index: {unique: true}}
});
userSchema.pre('validate', function () {
if ((this.isNew || this.isModified) && this.username) {
this.usernameCanonical = this.username.toLowerCase();
}
});
const User = mongoose.model('user', userSchema);
mongoose.connect('mongodb://localhost:27017/uniqueTest')
.then(() => {
// create two users
const user1 = new User({
username: 'EXAMPLE_USERNAME-1'
});
const user2 = new User({
username: 'EXAMPLE_USERNAME-2'
});
return Promise.all([
user1.save(),
user2.save()
]);
})
.then(() => {
// update username
return User.findOne({ username: 'EXAMPLE_USERNAME-1' })
.then((user) => {
user.username = 'EXAMPLE_USERNAME_UPDATED-1';
return user.save();
});
})
.then(() => mongoose.connection.close())
.then(() => console.log('script finished successfully'))
.catch((err) => {
console.error(err);
mongoose.connection.close()
.then(() => process.exit(1));
});
I hope this helps.