ActiveRecord::RecordInvalid: Validation failed: User must exist - ruby-on-rails

Question

I'm working on a issue with Single Table Inheritance. I have two different types of Users. User model and Trainer model, Trainer user should inherit attributes from the User model. I created a User in the rails console and everything worked. As soon as I attempted to create a Trainer I get the following error.
Rails 5.0.4
ActiveRecord::RecordInvalid: Validation failed: User must exist
Am I setting up my model associations incorrectly?
Here is my User Model
class User < ApplicationRecord
# Include default devise modules. Others available are:
# :confirmable, :lockable, :timeoutable and :omniauthable
devise :database_authenticatable, :registerable,
:recoverable, :rememberable, :trackable, :validatable
end
My Trainer Model
class Trainer < User
has_many :appointments
has_many :clients, through: :appointments
end
Schema for models
create_table "trainers", force: :cascade do |t|
t.string "first_name"
t.string "last_name"
t.datetime "created_at", null: false
t.datetime "updated_at", null: false
end
create_table "users", force: :cascade do |t|
t.string "email", default: "", null: false
t.string "encrypted_password", default: "", null: false
t.string "reset_password_token"
t.datetime "reset_password_sent_at"
t.datetime "remember_created_at"
t.integer "sign_in_count", default: 0, null: false
t.datetime "current_sign_in_at"
t.datetime "last_sign_in_at"
t.string "current_sign_in_ip"
t.string "last_sign_in_ip"
t.datetime "created_at", null: false
t.datetime "updated_at", null: false
t.string "type"
t.index ["email"], name: "index_users_on_email", unique: true
t.index ["reset_password_token"], name: "index_users_on_reset_password_token", unique: true
end
As you can see from my User model, I added the required :type column
Here is the schema for my client and appointment
create_table "appointments", force: :cascade do |t|
t.integer "client_id"
t.integer "trainer_id"
t.datetime "appointment_date"
t.datetime "created_at", null: false
t.datetime "updated_at", null: false
t.datetime "start_time"
t.datetime "end_time"
t.integer "status", default: 0
end
create_table "clients", force: :cascade do |t|
t.string "name"
t.string "phone_number"
t.integer "price"
t.datetime "created_at", null: false
t.datetime "updated_at", null: false
end
First I created a user in the console
User.create!(email:'ryan#test.com', password:'asdfasdf', password_confirmation:'asdfasdf')
Then I when on to create a Trainer
Trainer.create!(first_name:'Ryan', last_name:'Bent')
Trainers and Users should be associated. But I didn't think I needed add associations using Single Table Inheritance.

Answer 1

With Single Table Inheritance, one table must have all the attributes that any of the subclasses need (more information). So for your situation, you'd need to add the Trainer columns (first_name, last_name) to the users table as well, and then Users would leave that empty on the table and Trainers would fill them in.
If you want to keep the separate tables, what you are doing is no longer single table and would require some sort of joining between the 2.