Can't add Company (from separate model) to Devise User - ruby-on-rails

Question

First-time poster, many-time finder-of-answers on the site (thank you!). I'm using Rails 5.2.3, ruby-2.6.2 and Devise gem 4.6.2. I have not been able to get an answer to work, even though there are plenty somewhat related questions here, here, here and here.
When a new User signs up, I want them to select their Company from a dropdown list (already created) in the sign-up form. (Eventually, this will be an admin role, but that's beyond the scope of this question.)
I created a registrations controller and added code per a number of the previous posts. Update, I was not extending Devise as I should have as indicated here: Extending Devise Registration Controller. This is my new Registrations controller.
class Users::RegistrationsController < Devise::RegistrationsController
before_action :configure_sign_up_params, only: [:create]
before_action :configure_account_update_params, only: [:update]
def new
#companies = Company.all
super
end
def create
#companies = Company.all
super
end
protected
def configure_sign_up_params
devise_parameter_sanitizer.permit(:sign_up, keys: [:company_id])
end
def configure_account_update_params
devise_parameter_sanitizer.permit(:account_update, keys: [:company_id])
end
end
And created new files in views/registrations with new.html.erb and edit.html.erb that I copied the exact code from the devise/registrations views.
I updated my routes.rb file to include:
devise_for :users, :controllers => { registrations: 'users/registrations', sessions: 'users/sessions' }
My User model is:
class User < ApplicationRecord
devise :database_authenticatable, :registerable,
:recoverable, :rememberable, :validatable
belongs_to :company
accepts_nested_attributes_for :company
end
My Company model is:
class Company < ApplicationRecord
has_many :users
end
In the new user registration form, this works to provide the dropdown, but when I try to create the new user, it says: 1 error prohibited this user from being saved: Company must exist.
<%= f.collection_select :company, #companies, :id, :name, prompt: true %>
I thought this post would have the answer, but that appears to use Rails 3 and attr_accessible, which was deprecated in Rails 4.
I don't really understand what accept_nested_attributes_for :company does. The only thing in the Company model is the name.
Thank you in advance!

Answer 1

Welcome to StackOverflow.
In order to add more parameters to devise's sign up form, you'll need to sanitize the corresponding parameters using devise's sanitizer.
You should do that like this:
class ApplicationController < ActionController::Base
before_action :configure_permitted_parameters, if: :devise_controller?
protected
def configure_permitted_parameters
devise_parameter_sanitizer.permit(:sign_up, keys: [:company_id])
end
end
You can find more information about parameter sanitizing and adding custom fields in this section of devise's readme
If you also want to add a select field including all the existing companies, you should add a collection select:
<%= f.collection_select :company_id, Company.all, :id, :name %>

Answer 2

Got it!
To extend the Devise controller, follow the help here: Extending Devise Registration Controller
The User models must also be updated to include the optional: true because here https://blog.bigbinary.com/2016/02/15/rails-5-makes-belong-to-association-required-by-default.html:
class User < ApplicationRecord
devise :database_authenticatable, :registerable,
:recoverable, :rememberable, :validatable
belongs_to :company, optional: true
accepts_nested_attributes_for :company
end