class User < ApplicationRecord
  has_one :employee
end

class Employee < ApplicationRecord
  belongs_to :user
end

{
    "username": "my_user",
    "password": "12345",
    "employee": {
        "name: "my_name",
        "lastname": "my_lastname"
    }
}

class UsersController < ResourcesController
    def create
        user = User.create! do |u|
           u.username = params.username
           u.password = params.password
        end
        employee = Employee.create! do |e|
           e.name = params.name
           e.lastname = params.lastname
           e.user_id = user.id
        end
    end
end