Validate unique value of database using knex.js and validate.js

index.js

const assert = require('assert')
const validate = require('validate.js')
const db = require('./db') // instance of knex

// custom validation

validate.validators.unique = function (value, options, key) {
  assert.ok(!!options.table)
  assert.ok(!!options.column)
  assert.ok(!!options.db)

  return options
    .db(options.table)
    .where(options.column, value)
    .first(options.column)
    .then(row => Promise.resolve(row ? `${key} is already used` : undefined))
    .catch(console.error)
}

// usage

const constraints = {
  username: {
    unique: {
      table: 'users',
      column: 'username',
      db: db
    },
  }
}

validate
  .async({ username: 'someone' }, constraints)
  .then(() => console.log('ok'))
  .catch(err => console.log(err))