I want to make a user registration using a generated key, with only one registration per key. Using Devise. In my DB I will have a generated key that is not assigned to any user, so when a user is registering and using the key this key will be assigned to the user.

I have created the RegistrationKey model with the following migration:

class CreateRegistrationKeys < ActiveRecord::Migration[6.1]
  def change
    create_table :registration_keys do |t|
      t.uuid :key
      t.references :user, null: true, foreign_key: true

      t.timestamps
    end
  end
end

# registration_key.rb

class RegistrationKey < ApplicationRecord
  before_create :generate_key

  belongs_to :user, optional: true

  private

  def generate_key
    self.key = SecureRandom.uuid
  end
end

Updated my User model:

# user.rb

class User < ApplicationRecord
  has_one :registration_key

  devise :database_authenticatable, :registerable,
         :recoverable, :rememberable, :validatable

  validates_presence_of :registration_key
end

Permitted the registration_key param in registration controller:

# registration_controller.rb

class RegistrationsController < Devise::RegistrationsController
  before_action :configure_sign_up_params

    ...

  def create
    registration_key = params[:user][:registration_key]

    existing_key = RegistrationKey.find_by(key: registration_key)

    params[:user][:registration_key] = existing_key
    
    super
  end

  ...

  protected

  def configure_sign_up_params
    devise_parameter_sanitizer.permit(:sign_up, keys: [:registration_key])
  end

end 

Added registration_key input in my new registration view:

# devise/registrations/new.haml.html

%h2 Sign up!!
= simple_form_for(resource, as: resource_name, url: registration_path(resource_name)) do |f|
  ...
    = f.input :registration_key
  ...

I have generated a registration key in my DB, which I use to signup but I still get the error, I cant figure out what is wrong and how to fix it:

Log:

Started POST "/users" for 127.0.0.1 at 2022-01-16 14:05:28 +0200
Processing by RegistrationsController#create as HTML
  Parameters: {"authenticity_token"=>"[FILTERED]", "user"=>{"email"=>"test@test.com", "registration_key"=>"[FILTERED]", "password"=>"[FILTERED]", "password_confirmation"=>"[FILTERED]"}, "commit"=>"Sign up"}
"registration_key"
"7a573aef-2d60-4561-bc29-0965647449a4"
  RegistrationKey Load (1.0ms)  SELECT "registration_keys".* FROM "registration_keys" WHERE "registration_keys"."key" = $1 LIMIT $2  [["key", "7a573aef-2d60-4561-bc29-0965647449a4"], ["LIMIT", 1]]
  ↳ app/controllers/registrations_controller.rb:20:in `create'
Unpermitted parameter: :registration_key
  TRANSACTION (0.9ms)  BEGIN
  ↳ app/controllers/registrations_controller.rb:26:in `create'
  User Exists? (1.2ms)  SELECT 1 AS one FROM "users" WHERE "users"."email" = $1 LIMIT $2  [["email", "test@test.com"], ["LIMIT", 1]]
  ↳ app/controllers/registrations_controller.rb:26:in `create'
  TRANSACTION (0.7ms)  ROLLBACK
  ↳ app/controllers/registrations_controller.rb:26:in `create'
  Rendering layout layouts/application.html.erb
  Rendering devise/registrations/new.html.haml within layouts/application
  Rendered devise/shared/_links.html.erb (Duration: 0.3ms | Allocations: 114)
  Rendered devise/registrations/new.html.haml within layouts/application (Duration: 16.3ms | Allocations: 10451)
[Webpacker] Everything's up-to-date. Nothing to do
  Rendered layout layouts/application.html.erb (Duration: 22.0ms | Allocations: 13977)
Completed 200 OK in 314ms (Views: 22.6ms | ActiveRecord: 15.8ms | Allocations: 26287)