A Sequelize plugin to add isUnique validation to models

I needed to validate a nonpersitent Sequelize instance prior to saving it. Basically, when creating a user instance for my app, I needed to validate the email and username as being unique prior to saving the instance. After failing to find a plugin that would add this feature, I came across a thread on the Sequelize Github board:

Why is unique not a validator?

npm install sequelize-isunique-validator -S

var Sequelize = require('sequelize');
require('sequelize-isunique-validator')(Sequelize);

var User = sequelize.define('User', {
    name: DataTypes.STRING,
    email: {
        type: DataTypes.STRING,
        allowNull: false,
        isUnique: true,
        validate: {
            isEmail: true,
            isUnique: sequelize.validateIsUnique('email')
        }
    }
});

User.create({
    name: 'John Doe',
    email: '[email protected]', //this email exists
}).catch(function(err){
    console.log(err)
});

What you get is

[SequelizeValidationError: Validation error: That email is being used]
name: 'SequelizeValidationError',
message: 'Validation error: That email is being used',
errors: ...

When using sequelize.validateIsUnique() the first parameter must the the field to do the check on. For example, isUnique: sequelize.validateIsUnique('email'). In order for the validation to pass for an existing instance, the primary keys for that instance will be used to exclude values for that instance. Based on the previous example, the query it performed was something like this ( depending on your database):

SELECT count(*) AS `count` FROM `users` AS `User` WHERE `User`.`email` = '[email protected]' AND `User `.`id`  IS NOT NULL;

The second parameter allows you to define a custom error message. The default error message is: '{{field}} must be unique'. Using our previous example we could do something like this:

var User = sequelize.define('User', {
    name: DataTypes.STRING,
    email: {
        type: DataTypes.STRING