Mongoose.js Unique Validation - node.js

Question

NOTE: there's an edit at the bottom of the question:
Can I check the database for uniqueness using either a custom validator or a pre hook in a Mongoose.js model file. I am aware that I can check it in the controller, but I'd rather put it in the model file with the rest of the validators just for consistency.
I am also aware there is an npm package called mongoose-unique-validator that does this but I'm no fan of installing a library to do what should be one to five lines of code tops.
Mongoose also has a "unique" property that will throw an error if the item is not unique. But their documents clearly state this is not a validator. And the error it throws does not get routed the same as the validation errors.
Here is the relevant parts of the model file. This will check the db and if there is no dup then it creates the article but if there is a dup it throws an error but not a validation error which is what I want. If I simply return false if there is a dup it just ignores the validation and creates the duplicate article. This is no doubt related to Promises/Async. Here are the relevant Mongoose docs https://mongoosejs.com/docs/validation.html#async-custom-validators. And they talk about how the unique property is not a validator https://mongoosejs.com/docs/faq.html.
const mongoose = require('mongoose');
const Schema = mongoose.Schema;
const articleSchema = new Schema({
title: {
type: String,
required: [true, "Title is required"],
// unique: true,
// isAsync: true,
validate: {
validator: function(value) {
this.constructor.findOne({title: value}, (err, article) => {
if (err || !article) {
return true;
} else {
// return false;
throw new Error('Duplicate');
}
});
},
message: (props) => `Title "${props.value}" is already in use.`
},
},
content: { type: String, required: true }
});
EDIT: I figured this out, but it only works when creating a new article, not on updates. So the question is still open but the focus is on how to get it to work on updates. On update Mongoose does not treat "this" as the document object like it does on create. Instead "this" is the request object, and "this.constructor.findOne()" throws the error "this.constructor.findOne is not a function". Here's the revised validator:
title: {
type: String,
required: [true, "Title is required"],
isAsync: true,
validate: {
validator: async function(value) {
const article = await this.constructor.findOne({title: value});
if (article) {
throw new Error(`${value} is already in use.`);
}
}
}
}

Answer 1

Your validator function will only run the script and it not pass any callback or promise to mongoose, so mongoose assume that the validator return true and continue the process.
According to the document, you should return promise or use callback.
Promise:
validator: function(value) {
var here = this;
return new Promise(function(resolve, reject) {
here.constructor.findOne({title: value}, (err, article) => {
if (err || !article) {
resolve(true);
} else {
resolve(false);
}
});
})
}
Callback: (need to set isAsync: true)
validator: function(value, cb) {
this.constructor.findOne({title: value}, (err, article) => {
if (err || !article) {
cb(true);
} else {
cb(false, "Content is used");
}
});
}