sequelize-cross-database-updated-query.js

const product = await UsersDb.models.order.findOne({
  where: { id: 1 },
  include: [
    {
      model: ProductsDb.models.product,
      on: {
        // this is where magic happens
        order_id: Sequelize.literal("`order`.`id` = `products`.`products`.`order_id`") 
      }
    }
  ]
});
console.log(product);

// output
{ id: 1,
  user_id: 1,
  order_date: 2019-05-10T07:00:00.000Z,
  products:
    [ { id: 1, order_id: 1, name: 'oathkeeper' },
      { id: 2, order_id: 1, name: 'longclaw' },
      { id: 3, order_id: 1, name: 'needle' } ] }